What is Clinical Depression?

Can someone basically sum up what clinical depression is? for the past month or so i have been having random fits of depression. i have written down all of the things that have made me feel any strong emotion and to be honest, none of them alone are enough to do this to me. i think of myself as a very strong minded person but lately i have been kind of... "Bleh" if you have any idea what i mean...its gotten to the point where i have almost completely stoped eating. in the past 5 days i think i've eaten half a bowl of Ramen, and a small bowl of spaghetti which i was forced to eat. i am 16 years old and my average weight 2 months ago was about 125lbs. now my average weight is about 110lbs. i am not exactly sure what the problem is but my life isn't that bad. i need to know if its actually depression or a chemical imbalance. dont ask whats wrong because i'm not sure. thats why i had to write what i could think of down on paper because it all kind of blurs together and its hard to think. please just give me some general info and if you want, some advice on what i should do or the mind set i should be in cause the one i'm in right now isn't working too well.