Alright, here's the REAL definition of emo. It will be broken up since there are two definitions. There's the style of music, and there's the lifestyle.
1) Emo music is a sub-genre of punk. It is often characterized by high-pitched male vocals and lyrics in which emotion(hence the name) is expressed. The lyrics often have to do with betrayal, lost loves, "broken hearts", suicide/SMB (self-mutilating behaviors), and depression. Bands such as Dashboard Confessional were thought to have created the emo genre. See also: screamo.
2) The emo "lifestyle" has changed somewhat over the years. It started off with kids who were pretty much the outcasts. The guys were thought to be in touch with their sensitive side and they would wear rectangular framed glases, scarfs (much like those seen in Harry Potter), tight t-shirts, and occassionally tight pants. They would listen to emo music, and often be depressed. However, this has changed. Now, emo boys can be distinguished by their long (usually black and sometimes with a streak of another color) hair, which is cut in such a style so that it covers one eye or half of their face, and their skinny figures. They also wear eyeliner(not always), tight t-shirts, girl pants(sometimes), usually some sort of hoodie or hooded jacket, and usually sport black fingernails. Emos are usually depressed and are fragile with strong emotions. Some misconceptions about emos are that all of them want to commit suicide and all of them cut their wrists. Though many emos do this, not all of them do. Another misconception is that all emo guys are either bisexual or gay. This is not true as there are many straight emo boys and sexual preference has nothing to do with it. If you think that all emos have made-up problems, you are reading the wrong definition and should look at emo poser.
1)Bands often considered to be in the emo genre include My Chemical Romance, Fall Out Boy, Panic! At The Disco, Dashboard Confessional, Taking Back Sunday, and many others (most of them are rather obscure).
2) GirlA: The other day I took some totally emo pictures with my fingers pointing at my head and eyeliner streaming down my face. I'm like, such a camera whore. Then, I cut my wrists.
GirlB: You're a poser.
By Someone who actually knows