The thing about peer review is that there isn't much of an education process. Maybe you've published a paper or two and experienced the process as an author, and then you're asked to start reviewing for other authors. It's a bit like the telephone game - you mimic the reviews you received, maybe noting what you liked and avoiding what you didn't like. But that's often all you have to go on, and when you're just beginning a little advice might come in handy. To that end, the British Ecological Society has just published a pretty useful Peer Review 101 text. This should be required reading for new reviewers.