The word "depression" has two meanings. The first one is expressing disappointment. For example, if someone doesn't return your call or your favorite restaurant is closing. The second meaning is referring to a physical illness caused by a biological imbalance in your body, specifically, in the chemistry of your brain.
There are two basic types of depression: major depression which lasts at least two weeks, and the chronic dysthymic disorder, which lasts for a year or longer.