An entire subculture of people (usually angsty teens) with a fake personality. The concept of Emo is actually a vicious cycle that never ends, to the utter failing of humanity, and it goes something like this:

1. Girls say they like "sensitive guys" (lie)
2. Guy finds out, so he listens to faggy emo music and dresses like a dork so chicks will see that he is sensitive and not afraid to express himself (lie). He dyes his hair black, wraps himself in a stupid looking scarf, develops an eating disorder, and rants about how "nobody understands".
3. Now an emo guy, he meets Emo chick and they start dating, talking about how their well-off suburban lifestyles are terrible and depressing (lie)

from: link