What is depression?

I'm going in to counseling for the first time on Thursday, and while I know I'm not a complete and utter head case on the verge of hysteria, I do wonder what "it" is that makes someone "depressed." My best friend had to leave school two weeks in to the semester, and I guess that was probably what "pushed me over," but could it really be that simple? The dean at school says I need "perspective," because according to him depression is "cyclical." I believe it, but some thoughts on the subject would be appreciated.