I have up-voted Elizabeth Henning's answer, but to expand a bit for those unfamiliar with the concept:
Related term: "lunch and learn"
Generally held at or around lunch time.
Will often be considered as not part of work time. Whether they are optional depends on where you work - they aren't always optional (sometimes you're expected to attend a certain number per year for continuing education).
Often it is person within your institution who does the presentation - instead of a person brought in to present to you.
The presentation isn't always directly related to your (or their) work and may be completely unrelated.
Generally where I've worked it has been considered polite to attend and act interested - but my experience with them has been more at the "less than 20 people" co-worker/department level.
I have seen the frequency decrease in the last ten years or so, YMMV
Also note comments in other answers that contradict my answer