"Indie" stands for "Independent". They listen to indie music, and often use ridiculous words like "raw" and "organic" to describe it. They seem to think their music is different from everyone else's when in fact it is the same.
Indie kids wear clothes from second hand shops, have an unfounded fascination in things like cassette tapes and vinyl, and often wear thick rimmed glasses.
The thing that makes their music "indie" is a mystery. Any type of music can be indie- rock, rap, bluegrass, etc. As long as the audience consists of old plaid and tight jeans, it is indie.
Indie kids get off on listening to music that nobody has heard of.(often times, it is some random crap they found on myspace) If they tell you their favorite band, and you have heard of it, they have failed as an indie kid.
Eric: "I'm so indie, my favorite band has only 14 plays on myspace"
Tom: "I'm so indie, I listen to bands that haven't even been invented yet"
Normal person: "What type of music does your band play?"
Indie kids: "Oh yeah, my band is like new post-modern-pre-hardcore alternative with a hint of crunk and turkish folk. We're pretty raw."