Late form of *mostly* US punk, for the most part characterized by:
-gloriously filthy distortion on guitar and often on bass
-LOTS of feedback
-thick sound, usually in the lower octaves
-being generally raw and “unpolished” (but still good quality, mind you!)
-vocals that are either loud, mumbling, and unintelligible; yelled angrily and are, once again, unintelligible; or overly pronounced in a bitter, pissed off, or sarcastic manner
-lyrics that are often metaphoric and angsty, crestfallen, regretful, bitter, ironic, sarcastic, disdainful, symbolic, etc.
-lack of focus on looks/appearances and more focus on the music itself
-being against sexism, racism, and homophobia (and pretty much any other kind of discrimination)
-lack of emphasis on being “macho”
-being supportive of female involvement in music
Bands such as Green River, Mother Love Bone, Mudhoney, Alice in Chains, The Melvins, Stone Temple Pilots, Hole, Screaming Trees, Nirvana*, Dinosaur Jr., Smashing Pumpkins, Soundgarden, Silverchair, etc. (and trust me, there’s a lot more)
*Nirvana is not the first grunge band (Green River is more like it), nor are they the best one. They’re great and one of my favorite bands, but there are plenty that equal or outdo them. And for the record, Smells Like Teen Spirit is nowhere NEAR their best song, and Nevermind is CERTAINLY not their best album. Bleach is a far better album, and as for songs, Paper Cuts, Floyd the Barber, Scoff, Downer, Milk It, Pennyroyal Tea, and Scentless Apprentice all completely OWN Smells Like Teen Spirit.
**Disclaimer-esque-thingy: Although reading about different genres of music is informative and beneficial, the best way to learn about any type of music is to go buy the CD and experience it yourself (at full volume). And for goodness sake, please refrain from listening to it on those godawful devices known as MP3 players. CD players, boomboxes, and multiple-disk stereo systems are the way to go!
If you listen to the influences of your favorite grunge bands, you'll find even more awesome music.