Grunge is epitomised in bands like Nirvana. It conveys an "I don't give a fuck" attitude.
It is loosely tied in with rock and punk, but stands as it's own genre of music and style of dress. Instead of going out of their way to be the opposite of a "conformist", people with grunge style simply don't participate. They are generally the ones who play guitar, live for the music, let their hair get knotted, their clothes filthy and their attitudes nonchalant.
They just don't care, they throw on whatever. Kurt did it the best.
"Sweetheart, you should really shower and put some more thought into your outfits, unless you're trying to work the whole grunge style thing"