Emo means emotional. Basically meaning that emo kids (people who are emo) are sad. Somebody who is emo usually cries frequently, hates their life, hates their parents, etc.
Somebody is acting emo when they are:
1. being sad all the time
2. whining about life all the time
NOT EMO ATTITUDE: wrist cutting (just dumb)
Online: sad usernames looking like this: xXyour.name.hereXx
NOT EMO ONLINE: typing.like.this or TyPiNg LiKe ThIs (just dumb as well)