Avant-guard jazz fan: Sigh...
Many indie rock bands are characterized by the use of unique or less-popular instruments. Subjectively (since music is subjective), indie rock cds typically sound less "perfect" than mainstream music, which typically grabs the attention of indie rock enthusiasts.
Indie rock is a mixture of many other genres, including alternative rock, folk music, electronica, even pop. It also tends to be very positive music, despite popular opinion.
Others that are popular among indie rockers: The Flaming Lips, The Shins, Bright Eyes, The Decemberists, Wilco, The Magic Numbers, Mates of State, The Essex Green, Calexico, dios (malos), Clap Your Hands Say Yeah, Rilo Kiley, far too many to list. Find a Paste magazine and dig in.
indie although is very competitive and mos hipsters compete for who knows more "un-known" bands.
some indie rock record labels:
2.Underground, somewhat experimental, music. (modest mouse, sigur ros, the decemberists)
3. A tribe of people, hipsters, indie kids who all try to latch onto the next hip band before their friend does.
I know they're on a major label, but they are definitely Indie Rock!