In reality... (and I mean, reality beneath what MTV and the like will tell you)... emo is Emotive Metal - bands like Rites of Spring (and crap like that
). Emo used to be a term that people would call you if you were a loser.
Kids with white bags with multi-coloured dots that listen to Taking Back Sunday or Panic! At The Disco are not emo. Unless they are Korean*, female and have a sibling who has a child.
Bootnote: I usually use the term to describe the kids who go about thinking "omg I have so many problems, no one loves me", like everyone else, because that's what they understand. My previous post still applies if you take this into consideration.
* Emo is the rough approximation of how to say "Aunt" in Korean, apparently.