So there's this cool girl named Ellie Wilson, and for a high school English project, she tried to define "emo." She surveyed bands as well as just fans of music, and got some interesting results. Check it out here [and see if you can spot the quotes from yours truly!]. Good work, Ellie. If I were your teacher, I'd give you an A.
UPDATE: Looks like I sent a little too much traffic to Ms. Wilson's site. Keep trying, everyone, it's really worth the read.