Not actually a musical genre. Stems from the meaning of independent labels, which can apply to any REAL musical genre, from punk to metal and their subgenres, to underground hip hop etc. Bands such as Arcade Fire, Modest Mouse, The Academy Is, etc, are/were commonly labeled as Indie, but most bands that commonly fall into the "indie" label are actually just as easily (and correctly) labeled as art-rock or experimental.
Indie kids, however, are generally pretentious, stuck up (and by no means more educated/intelligent than anyone falling into another social group) copy-cats that think they know everything about music, movies, life, etc. One commonly sees them dressed up in earth tones with suspenders, sporting such "outdated" fashions as corduroy, and those slanted hats commonly associated with grandfathers and old-school
indie labels: anything that is not funded/ran by large corporations and multinational businesses, even more popular labels such as Epitaph and Fat Wreck-Chords would be considered independent labels
Indie kid 1: "Hey Chuck, did you hear that new Arcade Fire CD?"
Indie kid 2: Uh, James, Arcade Fire sold out man, they aren't indie. But those are some nice suspenders you have. Let's philosophize about the affect of cheese on society."