Its a form of Punk. YOu can't desribe Nirvana, or grunge for that matter. Have you heard Alice in Chains? Pearl Jam? Soundgarden? Its somewhat like that....Nirvana is probably the heaviest of the group though really.
You have heard a song by them, you just don't know it....
http://www.mp3.com/albums/11566/summary.html
Thats a good site for some videos on them. And if you are a member on Yahoo then go to
www.launch.com
And do a search for Nirvana.