To me, the definition of grunge is someone who just cant afford to buy those preppy clothes that everyone wears, and they wanna stick out a little bit. It's also for people who dont really care what they look like. Like not taking showers, wearing what ever's on your floor, and going to school with bed head. haha. You may interpret it differently, but thats just how i see it in my head.
A good example of grunge is Kurt Cobain. He never really cared what he looked like or what other people thought he looked like. Plus he never really could afford the clothes that other people wore. He grew up around not having to much money.