A genre' of music as well as a 'fashion' and part of an individuals personality. Most 'Indie' people consider themselves the gurus of music and that their genre' of music is the only one able to be appreciated as 'real' music.
Indie's give off a very elitist asshole attitude which is unbelievably annoying.
They claim to only listen to 'underground' music that isn't widely popular or common, which is contradicted by bands like Radiohead or artists like Adam Young from Owl City.
According to Indie kids, Adam Young is a sell-out?
Basically, if you're an artist who makes money, you are no longer considered popular amongst Indie kids.
Also, Indie people like to put themselves above the other social status of 'Emo' although they appear to be insanely similar, besides attitude. Also, 'Scene' kids seem to differ from Indie. Basically, you have to be a total douche bag to be an Indie kid who likes to put themselves above everyone else because they have a depressing home / school life and they've mastered the art of alienation.
In essence, an Indie = Emo / Scene without the whining emotion, and more of a stuck-up princess attitude.
Person: Hey, a friend told me you liked Modest Mouse?
Indie Kid: Not anymore?
Person: How come...?
Indie Kid: They're sell-outs and totally unoriginal...don't talk to me about 'good' music.
Person: ...I see... O.o lol?