Emo is a culture, just as jocks/nerds/gamers etc. If you ask this question, many people would say they're a group of children, wearing black, cutting themselves, listening to hardcore music blah blah... And that's just plain wrong. There's a difference between true emos and posers who's in it for the attention. Every emo kid has a different reason for "being" emo. Mostly because of previous life experiences, or just because of the fact that they don't fit in anywhere else. Emo is when you scream, cry, smile, etc when you feel like it, not caring what others think. And yes, most emos do harm themselves in one way or another, but many other teen cultures do, for example, when skaters are hurting, they'll drink themselves better. Google it, you'll find many different opinions and facts, the choose whatever sounds accurate enough. :)
An emo is simply someone who is considered by his or her peers as a cool person. It has to do with the way you dress, your swag, as well as how you handle people. When thinking of emo, a picture of the skater kid with tight jeans, long bangs and tight T-shirts comes to mind.