Emo is just a label(short for emotional) like nerd or jock etc, it doesn't really mean a whole lot about who that person is, just that they tend to dress in dark colours and listen to similar music. The myth that all emos cut themselves is really not true...a lot of teenagers do, but from all different labels.
Actually, alot of them do because they think they're supposed to. They crave to belong in a group and they follow the stereotypes to fit in. Oh cliques, you never cease to amuse me.
I'm in high school so I know a lot of emos...I know ones that do cut themselves but they have actually suffered from depression for a long time. I think the depression comes before becoming emo, and then maybe it just kind of fits to become one. I think the emos that do that stuff simply to fit in are the exceptions, not the rule.