Used to be used as an adjective, "emo" can now be a person. An 'emo' is a person (boys are typically more noticeable than girls) who develops a fake sense of overwhelming depression and that their life is all a waste. This perception is usually derived from emo culture but most of the time just music, whose lyrics very commonly state that life is all a waste and that suicide is a good idea. It's a sad, deeply pathetic but nonetheless very sad condition, and is even being considered its own mental condition alongside with depression.
An emo can physically be classified by dark, tight-fitting clothes, fabric accessories (scarves, belts, etc), and typically black hair that commonly covers one eye. Boys wear tight pants that come in at the ankle and very feminine-fitting clothing, while girls typically show a bit more skin and favor more gothic styles. The 'emo' stereotype is different from punk in that punk is much more colorful, and is different from gothic in the sense that their problems are more focused on them than the destruction of humanity.
Emo boys are treated more harshly than girls because of their feminine appearance. They are frequently called 'gay', and the word 'fag' is becoming commonly associated with them. Terrible, terrible hate is directed towards emo people because of their general pathetic appearance. On the flip side, emo is a a near cult; and if one emo meets another, they are befriended. There's only a rare number of people, such as myself, who are against being emo without hating the people.
And then there's 'Wanna-bemos.' These are the people who stumble around, saying "Omg, I'm so emo todayyyy!!!" These are the people who worship lead singers of emo bands, and wish they were 'only just as cool.' I personally believe this is even more pathetic than being within the actual emo stereotype. Why would someone wish to be depressed? It's a very confusing situation.
A young man in a black tee shirt with the title and logo of the band 'My Chemical Romance' plastered over the front, tight black jeans, dark converse shoes and messy, ebony-colored hair walks up to a girl of the same physical appearance. He looks up at her with only one visible, watery eye and passes her a simple "Hi."
She looks back with a sad smile. "Hi. What's your name?"
The young man blinks. "... Kevin," he says sullenly.
"My name's Ella. My Chemical Romance is the best band ever."
A girl with long, dark hair and completely black clothes bounces up to her boyfriend, who looks quite similar. "Hey, sexy," she croons, and stands next to him. "What are you up to today?"
"Soccer practice; it's Wednesday," The boyfriend says back, as if she were an idiot.
The girlfriend looks stunned. "But.. but we were going to see the Fallout Boy concert tonighhhttt!!! Pete Wentz is so hot."
Boyfriend shrugs. "Woops, forgot."
You notice that in the Emo example, both members were very quiet and obviously depressed. In the Wanna-bemo example, both members acted like the 'prep' stereotype; the only difference was that they liked emo music and dressed like such.