techincally it stands for "emotional"...here is the definition on wikipedia:
http://en.wikipedia.org/wiki/Emo_(slang)
emo guys wear eyeliner, straighten their hair, and have facial tattoos...heck emo girls do too. lol and they all wear SKINNY JEANS that are skin tight and a black t shirt. sorry if this is stereotyping, i think it's true tho.there is also the stereotypical emo pic (the girl is displaying it) where you hold the camera above you and look depressed. i google imaged so there are 2 pics, 1 of a guy and 1 of a girl.