I'm not a medical professional. This response is based only on my personal experience.
From what I understand, there isn't an exact enough definition to give a simple answer about what it is. This is mainly due to there being so many different forms as a result of both physiological and environmental factors.
Since depression is so strongly correlated to the concept of suicide, I used to think of it only as a pervasive sadness (which it can be), but when it showed up in my own life, I didn't even realize I was depressed until I started thinking about suicide myself.
When it hits me, I turn into a zombie. I block out everything and cease to experience novelty or meaning. I lose all desire to do ANYTHING. It's hard to even get out of bed.
Here are two things that have helped me:
1.
If you aren't in the depths of despair, it can be hard to fully empathize with someone who is. If you've NEVER been there, it's impossible.
So even someone with good intentions can give you shitty advice or make you feel worse (through no fault of their own).
This guy really gets it, and has a way of resonating with me when I'm in those dark places.
2.
http://deoxy.org/egofalse.htm
The term "ego" is thrown around a lot, but if you ask someone what the definition is, you'll almost always get a different answer.
This is an interpretation that makes a lot of sense of me. Reading it for the first time triggered a paradigm shift on par with a heavy psychedelic trip.
Good luck man.
Don't give up. Summoning the energy to try to overcome depression is in and of itself a huge step in the right direction.