An emo is a nerd/geek with a specific fashion style and specific tastes in music. They largely listen to "emo" music, which is often rock with some screaming over the top or some weird electro beat with a singer who only spurts vile, vulgar subjects such as sex, drugs and violence.
Emos like to dye their hair black. Girls like to wear extensions and put random streaks of crazy colours in their (often) dyed black hair, and both genders grow a long fringe over one side of their face to hide its ugliness. Other hair colours may include purple and bright red.
Both genders wear tight-fitting clothing and band tees.
They are social rejects who decided to try and cover this up by conforming to a "new style". They are a cheap rip-off of the 80s "New Romantics" sub-cult and the Japanese visual kei look.
Oh and ignore the people who are telling you that emos are "depressed" and hate their lives! They are not depressed, and there is nothing about their lives to hate. Their parents feed them, buy all their "cool" band tees, buy their concert tickets and put a roof over their head ... they are whiney, teenage brats who give others of their age a bad name. The ones who have the balls to cut themselves make "chicken scratches" in order to fit in with their peers. They are a DISGRACE to people who are truly mentally ill and depressed.