I've noticed that this is a term I've thrown around quite a bit in my posts.
"Emo" has no real definition, other than a style of music. Originally it was a shortened term to describe someone who was "emotional:" sensitve, shy, introverted, and/or angesty- the stereotypical listeners to emo music.
In time, emo came to be a label for people who dress in black, have bangs covering one or both eyes, wear black horn-rimmed glasses, have facial piercings, listen to "screamo" music, and write poetry involving dark themes. These traits, in turn, lead many to assume "emo" kids are depressed, suicidal, and self-harm. Like any label, no one actually calls himself emo- just like no athletic guy is going to call himself a jock!
Labelling = stereotyping. Most people who may look emo never have suicidal intentions, and many perfectly "normal" looking people self-harm.