Used to refer to the breed of teen who listened to(and "related to") soft punk music, dyed their hair black, made a habit of inflicting pain to themself (such as the sliting of the wrists), wear black stovepipes that are too small, dress entirly in black, and act overly moody/unloved;
Now its been so overused that anyone who has the slightest fringe, listens to any type of rock music often, or wears any one piece of black clothing is stamped with title emo.
You don't realy even need to be over emotional as the name suggests or listen to the music (which in most peoples opinion are the two main emo propeties) anymore to earn the title.
1.) "You're an emo cos if those jeans were black they'd be emo jeans."
2.) Jim: "Have you heard the new blink 182 song?"
Bob: "Omg, i didnt know you turned emo."
3.) "You're such an emo, why did you get a fringe?"