First, I have to apologize for my extended absence from both this site and my blog... I've not only been experiencing some career path type crises, but also studying for, taking, and hopefully passing comps. 
Comps, or comprehensive exams or qualifying exams, are what students in Ph.D. track programs take at the completion of all the course work (usually) or around the end of the third year in the program. From what I know of them - so mostly in applied psychology fields - they literally encompass anything you might have learned in a course (or were expected to learn) or what might have been offered in courses you didn't take, but could have. It's the entire subject matter that you are getting a Ph.D. in... this includes history, current issues, definitions, measurement and procedural knowledge... literally everything you can think of. 

While the process and timelines differ by program and discipline, the one thing they all share is the unbelievable amount of stress and pressure associated with them. You study for at least a semester, if not more, usually while continuing to take classes. There is no study guide (or guide really at all) and there's virtually no way you can ever feel fully prepared for them - after all, you could not possibly have read every article, every book, every blog, every conference paper or presentation, attended all conferences and presentations... at some point, you have to just be confident that you know enough to take on most questions. Basically, it's terrifying and in many ways, the opposite of what any Ph.D. student is used to feeling, at least in preparing for any school work or exams. 

The other part of this whole process that's in many ways unique for doctoral students.