I don't understand all of the categories and sub-categories.
It's true, all of the categories can be really confusing. Basically according to the DSM-IV, the main categories of depression are Major Depression, Bipolar Depression and Dysthymic Depression (also known as minor depression). All other categories of depression are sub-sets of one of these three major categories. So really, there is no difference between atypical depression and major depression because atypical is actually a subset of major depression; it is a type of depression that can accompany the diagnosis of major or minor depression. Part of the reason it is called atypicaldepression is that symptoms can be the opposite of what you might expect; namely over-eating and over-sleeping. Read more at atypical depression.