People have said this to be many different things. Well Im pretty sure you are all wrong...
"Emo" is NOT short for emotional, it is short for Emotive Hardcore. Yes. That is a kind of music. It first came out in the mid 80s and died in the late 90s, what we have today is "scenester".
We do still have what you might call "Emo music" Bright Eyes perfected it, Dashboard murdered it, making really really bad attempts of being an Emo band.
Then there are the kids who think they are emo....
Uhm.... these kids listen to screamo, not emo.
Im so emo, I go home and cry while I listen to my shitty Dashboard and pretend the songs are about my life. LOSSSEERRR