Grunge is a movement of 'alternative' sub cultures into a mainstream one. Much like Punk was, and RocknRoll and other genres/subgenres.
http://en.wikipedia.org/wiki/Grunge_music
Here is a good read
Semiotically, it possessed the very similar themes to Punk, but lacked really in its politicism/formation into a socio-political, and was really a way of pluralising the rock scene.