Are PhD students generally invited for such reviews? I was under the impression they would prefer someone with a Doctorate degree or more experience in the field.
What are the advantages/disadvantages of accepting to review?
The general discipline that the paper treats is related to my field of study but my active research is not necessarily related. Is it still OK to accept the invitation to review?
It is not unusual that PhD students get invited as reviewers, after all a PhD student will likely be a true expert in the field of the PhD. If you are a student during your first years of study, the request may be a little premature, generally speaking but if your are in your final year then it will be a good experience. What you need to consider is whether you can provide an insightful review of parts of or preferably the entire paper. You should have a sense of why you were invited, i.e. why your expertise may have been asked for.
Peer review is a vital part of the publishing process so getting experience of reviewing other's material is very worthwhile. If you continue in academia this will be expected of you so you will have to start sometime. You can definitely add reviewing for journals in your CV, not mentioning what you reviewed but certainly for what journal you have reviewed.
If yo have not done a review before you should probably ask someone (or preferable more than one) more experienced within your field for a brief outline of what should be included in the review and how to formulate the review.
I would just add to Peter's answer couple more remarks:
It is important to write reviews. I can tell you that the list of names of people who don't do reviews is, at least amongst people I know, a "public secret". I mean, people who don't do reviews are known for it and it's certainly a negative thing.1 On the other hand, if you reject because you don't feel strong enough to do it, that's fine. Still, there has to be "first" once.
If it's your first review, tell that to the editor once you decide to do it. Just a brief mail: I have recieved the preprint and I will review it. However, I would like to bring to your attention that this is the first review I am writing.2 After all, in many cases (especially at conference reviews), you have to choose a "confidentality score" from 1 to 5, which exactly says how strong do you as a reviewer feel considering the review.
Discuss with your supervisor. It's surely ethical to ask someone close to you for opinion/help, so don't hesitate to approach him if you feel so. You may agree with him that you read the article yourself, mark what things you consider problematic, and then he helps you classify which things are crucial and which are not, and how much positive or negative the review should be. After all, your supervisor is not only your research director, he is there to help you (but not to do your job) with all parts of the scientific work.
1 There're people who reject reviews in Elsevier and Springer and some other publishers' journals, because they don't like the fact that these companies profit from it a lot. That's probably fine, too.
2 It's in general good practice to reply to the editor and say whether you accept. Unfortunately, not many people do it.
- Yes, PhD student often have more time available than Faculty and they are actively keeping up with the literature themselves.
- Disadvantages - a good review takes quite a bit of time. Advantages - it's useful experience, you can add it to your CV, you will learn a little bit more about active work in your field.
- Yes, that's fine.
In terms of writing a good review, you will find a lot of helpful advice from any Google search. The one thing I would add to that is that (at least in Computer Science) it is sometimes quite easy to spot reviews from PhD students as they tend to be harsh and unhelpful. As a student yourself you are probably used to getting a lot of feedback on your work, and that is good. Don't be tempted to take your frustration out on the poor person who has written the paper you are reviewing.
Remember that at least part of the purpose of reviewing is to increase the quality of work in the field. If you review a paper which is rejected, likely it will be submitted elsewhere. If you accept the paper, the authors will improve it before final submission. So the purpose of the review, apart from quality control, is to tell the authors specifically how to improve their work. Avoid being vague in your criticism and avoid taking an unprofessional tone. Even if reviews are "blind", write as if the authors know you (and chances are that at some point you will meet them). Think about how you would wish your supervisors to give you feedback, and take your own advice.
By far the best reviews I have ever had have not been the most complimentary ones, or reviews from the best places I have submitted to, but they have certainly been the most helpful. They contained comments like "X is a poor presentation of the data, use the technique mentioned in paper Y". Or, "the author has used technique Z, this is outdated and should be replaced with W". The worst reviews I have had may well have been correct in what they said, but they have also been rude and unhelpful. For example, "X is not novel" is a fine criticism to make, but to be useful you need to say where X has been done before.