"Emo" is a genre of music, a 'branch' of the Punk genre, if you will.
It is generally softer and more emotionally driven than Hardcore, though still keeping some musical elements of the genre.
Of course, with most genres that become popular, come bands and artists who generally much less talented, stale and arguably capitalizing on it's success. To name a few - Aiden, My Chemical Romance and Hawthorne Heights.
The musical genre "Emo" has absolutely nothing, whatsoever, to do with fashion. Although, from such a tight-knit scene grew a similarity in clothing and hairstyles, which has been confused as "Emo Fashion" (however more accurately described as "Scene Fashion"). This led to the stereotyping and labelling of anyone who listened to the music and/or dressed similarly to "scene kids" as "emos", as one would label people as "jocks" "preps" "goths", and so on.
Increasing mainstream airplay and discussion sparked 'outside' interest in this scene, giving birth to (as 'authentic scene kids would say) "poser emos", blowing the stereotypes way out of proportion and labelling the "emo trend" as the hottest new fad for young people today, much like gansters and surfers ones were.
Currently, the socially accepted meaning for the word "Emo" is an angst ridden teen, who's outfit consists mainly of black, popularly "skinny leg" (tight) jeans, with a large fringe covering on average one third of their face and self-harm scars across their wrists and arms.
Comparing this with the aforementioned, musical roots of the term "Emo", I doubt there's much hope for the salvation of this original idea.
Really, I'm quite shocked at some of the comments posted here, colloquially - "emo bashing". You should really know better, as I would assume you know what it's like to be outcast. What good comes from transferring this pain on to someone else? Shame
that'd be emu.
