Answer:
Anger is not depression because everyone gets angry at one point. You can be angry for a second and then forget about it and change your mood. However, when you are depressed it is usually for a while and not so easy to just forget about and change your mood, as quick as you would if you were angry.
2nd Answer:
There is what we call anger, which is really feelings, and not an emotion. True anger, the emotion, has usually been considered to be the other face of depression (and vice-versa). These days, many psychology professionals would say about true anger and depression that their owner has a chemical issue in the brain, and that these are not 'self' generated issues.