There is a lot of conjecture about the nature and reality of depression in the modern world, and I daresay a lot of people diagnosing themselves with it who perhaps do not truly qualify. It is not a "blue" feeling, it is not merely a low point. It is actually quite hard to describe, even for someone as verbose as I tend to be. I tried to recently, in a blog and an image called "the Pull of the Tidal King."
This fictional ruler has grown more and more real for me over the last year - as a way to describe the experience, he is as close to an accurate description as I can get. Image being stranded on a small island, alone, with little to do or see. You try to build a life on this island, but far across the sea, there is an unseen ruler, a king, who can command the tides, and, without warning, send a force of waves at your island so powerful it will knock anything you've built down, and temporarily drown you. When his waters recede, you need to do it all over again - to find the strength to rebuild your life.