http://www.upworthy.com/what-is-depression-let-this-animation-with-a-dog-shed-light-on-it?g=3&c=upw1 This link is to a good video that gives an explanation of what depression is from the point of view of the one who is dealing with it.
I have known many people over the years who have dealt with the Black Dog in varying degrees. I have even had to deal with the Black Dog at times over the years myself.
Right now I know someone, that I care very much about, dealing with a very big version of the Black Dog. But he has yet to get to the point of really recognizing it and getting help dealing with all his issues relating to the depression and the other stress factors in his life that are making the depression worse. Last winter he was just depressed and dealing with the lack of motivation and short temper. Now, with the added stresses of other things, he is in the extreme negative world view stage about everything.
It isn't easy being around someone who is going through this...