At my university, most lecturers and professors use slides in teaching. This is true for most engineering, mathematics and science courses. I am more comfortable with doing the board work and then sharing my hand-written lecture notes with the students. From my personal experience, I feel that the latter is a better way of teaching.
So, what are the advantages of using slides? Why do you use slides?
Secondly, does the answer to the above question change if you were teaching graduate students?
I am a student and I much prefer lecturers who write on the board than the ones who use slides.
Profs using slides generally go into loop mode where they have the objective of going through all the slides before the end of the class. As such, profs tend to go in a very fast pace.
Writing on the board brings some dynamism to the lectures. The lecturer tends to pace himself much better, and students are then more encouraged to ask questions as they go along.
Also, having students write notes help them become more active and aware during the lecture.
In my experience I've seen lots of profs who just read the stuff from their slides without elaborating any further. The students then tend to fall asleep since they know the lectures are basically in the slides.
Also, I have the impression that profs using slides are lazier in their teaching, in the sense that they don't even review the slides they are about to present before the lectures.
Advantages of slides over boardwork? Laziness, clean hands, digitally distributable product that represents all the work you didn't actually demonstrate in class.
And... those are also the disadvantages.
Board work is usually more engaging, and at least the classes I've taken and given, board-based classes kept the students (and the instructor!) mentally and verbally engaged in the material. Its naturally more exciting to watch an animated instructor write things out, check himself mentally for a moment, subconsciously sharpshoot him yourself as he moves along, etc. than to just listen to what may as well have been a recorded session flashing by on screen.
People who write on boards tend to only write the important bits on the board, and have prose handouts full of other details for reading and reference later. The constraints of live board work perform a magically precise editorial function which forces the instructor to either be concise or fail. Handouts retain every benefit of the slides but without exposing the instructor to the temptation of drifting into a passive or mentally absent teaching mode.
This is particularly important in graduate or corporate classes where the material may be new, originated by the person teaching and not yet fully baked (a wonderful situation where the gallery's questions actively deepen the instructor/researcher's knowledge right then -- is there any greater ideal?). People who write on boards usually have either rehearsed their material or know it intimately enough to pace through an essentially hands-on class as they go -- even (or especially) in the case of new research. This is as good as it gets for live instruction!
And this leads me to the worst thing about slides...
The laziness permitted by slides is paradoxically compounded by the temptation to be verbose in the slides themselves. This is the worst form of detached instruction and often devolves to the point that the "instruction" consists of an underprepared instructor who feels prepared (he's got his slides, right!) essentially reading whatever is written on the slides to the class. In these cases equal time spent with a book in a quiet place benefits a student more than having suffered through the presentation. Such sessions are a sad satire on modern pedagogy; but satire is not what your students came for.
The points above also happen to be relevant in organizational briefings. The cardinal rule is that the person doing the talking is the class, not the board or slides or other AV assistance. Someone with something to say who knows how to say it doesn't require anything but his energy and voice -- the other stuff is either enhancement or distraction.
Slides can be great to force you structure your lectures before class and actually stick to them, and they are always more legible than writing on a board (plus less messy if you have to deal with chalk rather than a whiteboard).
That being said, the slides remove some of the dynamism from the course in that they have students think less critically than when watching you work with a board. Part of this disengagement simply comes from lighting, but another part comes from the way that slides can't easily be amended during a lecture.
The most fun aspect that I've noticed when I use boards is that the students who are taking notes write down everything that I write on the board. Those notes are much less structured than what I could produce with slides that they would later download, but the actual process of writing out the notes keeps students mentally engaged as well as promoting a discussion rather than just a lecture.
Advantages of using slides:
First of all, let me say that I think that there are lectures that are better taught with slides (e.g. more applied lectures) and lectures that are better taught on the board (e.g. most math lectures).
So, what are the advantages of using slides? Why do you use slides?
The most common (but imho the worst) reason is probably: because it is easier and less work in the long run. You can concentrate on what you want to say and you can easily reuse your slides from last year. You can also just upload your slides which is less work than writing down everything that you have written on the board.
It is easier to implement pictures/sound/videos/... what ever media works in your context. For example you can show data, output or interface of a program, how a plant looks like, ...
Students don't have to copy everything from the board and have more time to concentrate on the lecture and just make their own notes on the slides and fill out the blanks you left for them to fill. In addition there are no handwriting issues.
You can do more material in less time which is, when used incorrectly, a disadvantage but can be extremely useful if the material is very easy or if you just want to show some interesting applications/examples.
And ,you can still use the board for things that have to be done slowly, e.g. proofs, and to keep things less static, e.g. if there is a question you can just switch to the board.
Secondly, does the answer to the above question change if you were teaching graduate students?
No. Graduate lectures are more often in the category that I personally would present at the board (more proofs and complicated things) but that has nothing to do with the students, just the things I want to cover in the lecture.
I use slides as
However, my personal preference for any "proof" sections of the course is never to just use slides, but to use both slides and board. The slides hold the statement and maybe a key step or detail that's a pain to write down. The steps of the calculation are done on the board. This can be a pain in rooms where the projected slide overlaps with the board, but can be made to work in my cases.
I have used the same approach for UG and PG teaching without change: concepts and figures on slides, details on the board. The only difference I find, as a rule of thumb, is that in the graduate case it's tempting to give more details on the board as the concepts are harder and the steps involved longer. Using the slides helps me resist this and keep focused on the big picture, but the slides still tend to be shorter.
I teach courses written by other people. Thousands of trainers around the world teach the same courses. Using slides generated by the courseware authors gives us consistency and repeatability. Our courses prepare students for certification exams, so the course content must be consistent.
I do, however, also use whiteboards, mostly for scribbled quick diagrams. If the diagram is complicated I'll create and make it available electronically.
It is quite controversial whether slides are better than a board, and I think this also depends very much on what and how you are teaching. But you have asked specifically for advantages, so I will try to focus on those. I will, however, mention caveats.
As Shawn says, slides can work as a very nice conceptual map of where you want your lecture to go, compared to just going up to an empty board with a chalk. The caveat is that you can simply prepare a few sheets of handwritten notes, and use these to accomplish the same goal.
It is much easier to pace yourself, both because the slides serve as a detailed outline and also because you can use the number of slides as an estimate of how long you will take. (but this can also be done by preparing notes, counting the pages or doing a practice lecture with a stopwatch)
You can return to previous slides easily or peek ahead as needed without having to erase the board and re-write everything.
There is no risk of students having trouble reading your script (unless your slides are truly pathological), everything is clear. Of course, you could just write legibly.
Typing is faster/less tiring than writing for many people. Perhaps it takes you 50 minutes to type up everything on the slides you will use, but it would take you 70 minutes of lecture time to write it all out in class. Plus, all the time is invested before the lecture, so you can devote all your lecture time to lecturing, instead of writing.
On that note, with slides you can focus on your lecture without being distracted by writing (eg. which part of the board to write the next bit on, how big to write, how to lay it out, whether to erase some parts first). This may sound silly, but for instance my script is beautiful on paper but hideous on a board - every time I must use a board, I always feel very insecure with how ugly and not-neat my writing and drawings look, and this is very distracting to me because it makes the usual public speaking anxiety that much worse.
Generally, the maximum speed you can write legibly at is not guaranteed to be greater than the optimal speed of covering the material. With slides, there is less danger of having to spend a long time writing out lengthy material that can be understood very quickly, but only once all of it is displayed/written. But conversely, if you are lecturing faster than you can write, how will students keep up with their notes?
You only need to prepare slides once, and can reuse them with minor updates. If you tend to write on the blackboard, you will have to write all of the material again and again every time you teach the class (although you can reuse your notes).
You can (and should) speak information which is related, but not identical, to what's on the slides. Often slides have more basic, fundamental points as you develop more complex ideas verbally. In this way, students can easily solve problems like "what does that new term that the lecturer just used mean again?" and follow more easily. You can essentially have 2 parallel threads running simultaneously, and students can switch back and forth between them depending on which one is easier to follow. Imagine you are teaching archeometry to a class that is half chemists and half historians. During the chemistry slides, the chemists can listen to you talking about the intricacies of the chemistry, while the historians focus on the basics from the slides. Then during the history part (eg. metallurgical developments), vice versa. But it can be argued that such heterogeneous teaching is not a good idea in the first place.
The slides can be given out to students for study in their own time. (but so can scanned notes, photos of the board, video recordings of lectures, etc)
If slides are given ahead of time, students can print out slides, and significantly reduce how much writing they need to take by writing down only things which you say that are not on the slides (because they are better explained verbally, or because they came up after a question, etc). The caveat is that writing helps retention, so this may actually impair students' ability to learn the material on the slides.
With slides, you can display arbitrary images. For instance, complex patterns that would take time to draw on a board, plots that are hard to draw just right, photographs (both ordinary and technical such as micrographs), busy figures with many intricate features (sometimes the figure simply cannot be any simpler). But, you can just print handouts with the pictures, or use a good old fashioned slide projector.
You can include things like animations, video and audio. Animations are useless 99% of the time (although they may help at times). Video and audio can be invaluable in some classes, for instance videos of behavioral psychology experiments or audio in a music theory class. But then again, you can still teach your class on a board and only go to slides for the audio/video.
A bright rectangle of light in the middle of a dark room will focus everyone's attention on it. But on the other hand, it also makes it very easy to zone out and catch up on some quality Z time. (plus, if you darken the room it will be hard to write notes)
It is possible to name many advantages of using slides, especially if you don't mind very specific advantages. However, most aren't real advantages, in the sense that you can have a lecture that's as good, and probably better, by:
If you follow these, there are very few real advantages of slides, and many disadvantages (distraction, restricts your freedom of drawing and writing to PowerPoint's formatting options, not effective for learning). This is, of course, if your goal is to have a very effective lecture.
Sadly, often people aim instead to spend as little time and effort as possible on the lecture. In that case, PowerPoint will produce a mediocre lecture, while writing on the board will produce an awful one. Too often people take the idea that "old fashioned teaching on the board is better than slides" and then think they can do a good job with inadequate preparation by just teaching from the board instead of using slides. They end up rambling on and on, and after the lecture all the students say to each other, "What the hell was he going on about up there? I didn't understand a single thing!" (unless this happens to be an extremely talented teacher)
On the other hand, it is impossible to lecture from slides with zero preparation (since you do have to make the slides at least) so it forces some semblance of coherence on you, although not much. So, if you are going to do your students a disservice by not preparing adequately, you are better off with PowerPoint (but only in the sense that you are better off losing a hand than losing your arm).
I teach math - mostly to undergrads. As of late I have started using slides more and more. The reasons (and my rants):
Edit: Having said all that I do agree with the point of another answerer that it is easy to make the slides too polished, and only present the end product - a solution, a proof, whatever. This may be a problem on those occasions when the journey is more important than the destination. I am trying to learn how to capture that on the slides, too. At least the most scenic points. Also, I still like to give the students who do show up something extra. In those auditoriums, where the video projector's screen won't roll down in front of the whiteboard/blackboard I can do both.
From the perspective of a student, I have seen good and bad teachers, irrelevant what method they used (board or slides).
However, I never liked boards. You need to copy everything from the board. That is, you concentrate pretty all of your brain time on copying (depending on the teacher's writing speed which is usually quite high). This might be a good practice in your early school life, but in higher levels of education we can assume that students know how to write.
The advantage of using slides is that students can spend a small amount of their time on writing (additional notes) and most of their time thinking about the content. Hence, allowing them to connect and understand the information they hear and ask interesting questions.
For me, that's the critical and important advantage. If I want to read, I read a book. If I go to a lecture, it should be interactive. I should be able to ask questions. And answer questions posted by the teacher to the class!
In my opinion, it does not depend on the level of education. You should learn how to think and ask questions early on.
Another alternative is to use a tablet device with a computer projector to hand write notes. This has the advantage of slowing the lecturer down (just like writing on a black board) while being visible to a very large audience and easily recorded by a lecture capture system. I've switched to this approach in recent years and find that students really prefer it to prepared slides (which just fly by too fast.)
Not huge, you have more freedom to respond to students responses and questions with board work while slides tend to have a better structure.
If the class is looking a little lost it's easy to expand on something on the board but you can't make more slides on the fly.
If you do choose board work please do provide handouts or similar with the actual material you'll be scribbling up on the board.
when I was a student I could either be writing down what you just said or I could be thinking about what you just said. Your choice.
If you try to force students to scribble down everything while you're talking they're going to be thinking "damn, where's my spare pen, this one is going dry" not "Hm. I wonder how that principle applies to...." when you say something.
I use slides when I teach because:
It forces me to prepare for class in advance. (This is usually not a problem for me, but I had a professor in undergrad who only wrote on the board because he didn't have a lesson plan or anything prepared for the day. I vowed to never ever teach like that!)
I have time to articulate my thoughts on my slides.
The information that I want the students to know is on the slides. That means that EVEN IF I was lecturing too fast or a student couldn't take notes fast enough or the student couldn't read my handwriting or a student couldn't come to class, it doesn't matter. Because all the information is on the slides and I provide them with digital copies.
I get way more joke opportunities with slides and I can keep the students engaged.
It is, of course, possible to use slides in good ways and bad ways, just as it is possible to use blackboards/whiteboards in good ways and bad ways. I hardly ever use slides for reasons I document here:
http://okasaki.blogspot.com/2008/01/why-i-dont-use-powerpoint-for-teaching.html
But you asked for ADVANTAGES of slides.
One is for showing pictures. For example, if you wanted to talk about brush strokes in the Mona Lisa, you probably wouldn't try to duplicate those brush strokes on a blackboard. Many fields have plenty of pictures to show. (But note that I'm not talking about gratuitous pictures like 99% of clipart and stock photos.)
A second advantage can be when the presenter has a terrible memory or otherwise gets confused easily. The slides can be helpful reminders that keep you on track (but written notes might work as well). One place where I run into this situation is if I have to give the same lesson more than twice in quick succession, when it can be easy to become confused about whether you've made a particular comment during THIS session, as opposed the same presentation an hour ago, or the one an hour before that.
A third "advantage" is that slides made by somebody else can be a great timesaver for a lazy teacher. For example, some textbook publishers offer slides to go along with the textbook. So if you are a teacher who wants to put very little time into a class, using the premade slides can seem like a great choice. As a less perjorative example, I have sometimes provided slides to somebody that I was asking to fill in for me when I was sick or on a trip.
Edward Tufte has an essay on "The Cognitive Style of PowerPoint: Piching out corrupts within" (link). As you may expect, it criticizes slide show style in favor of hand outs, speeches and non-bullet listed slides. The essay is not a direct answer to your question but I think it contains relevant points.
Powerpoint corrupts absolutely ... but you can write slides that help students memorize what is the main point you are talking about. Then I use lot of board space to illustrate what I am saying. I taught once in Sweden and was very happy: there were boards all around the room so I could keep important drawings and assertions still alive and well! (I teach programming and programming is an art and what you say should be tailored to your students way of thinking so pre-cooked slides are not extremely detailed but often show images intended to illustrate a problem and help students memorize a path to their own re-engineering of the topic)..
Some great answers contributed so far. I think slides are great for illustrative purposes and to give some kind of structure to the lecture. However, there are certain subjects that are better done on the board-for example mathematics. On the other hand, subjects like biology are better illustrated on a slide. You can use slides to illustrate graphical displays, videos, etc (In my experience most lecturers don't use enough of these anyway). The pace of the lecturer is more difficult to control when using slides compared to boards. Using boards encourages students to ask questions and this makes the class more interactive-there's just something about approaching a blank board with creativity. If you have a bigger group, it may be more appropriate to use slides as students may not be able to see a board from afar-however, electronic boards would be a solution to this.
All in all, I think slides are often used inappropriately and a combination of the two won't hurt in most cases.
The reason I use slide in my gen-ed science classes is
...wait for it...
to get the students to engage.
This runs counter to much of the advice that you see in other answers because slides by default make it very easy for watchers to not engage.
I'm doing what PER people call "active engagement" in these classes and it takes the form of making them interact with me and their peers on the topic of the day. The slides are so that I can quickly put up question that will be the focus of interaction sessions five or more times a class: I force them to interact with me through class polls, answer cards, and direct questioning; I force them to interact with each other by asking them to convince one-another of their answer to these polls with there is disagreement and circulating to hear how they are approaching the problem.
This is not without it's costs. If I have misjudged how much the class will actually get I won't finish my material that day, or will finish early. I haven't been able to get this working to my satisfaction in classes other than gen-ed sections, though I am trying to do a little of it.
So, the slides aren't really the focus of the instructions. It's the interactive questions that make up the bulk of the class time, and the slides help me put them up without a lot of fuss. Of course, I also put the structured part of the lecture on the slides because I am going to have the lights dimmed and the projector on.
I also only lecture for about 60% of these classes, turning the later part of class over to activities, group exercises, and directed follow-up investigations. I can circulate during those times and try to help out students who were floundering during the lecture part of class.
I've had less luck porting this style to my deeper and more math intensive courses, and mostly lecture from the board for those classes, though I do fall back on the questions-on-slides style for some foundation building days.
