Nirvana doesn't fit into the category of music and neither do nay other band because everyone has a different meaning of grunge or punk or all those names you use to classify bands or an era under. Nirvana don't sound like any other band and they're an awsome band (the best in my opinion).
Greg Ginn said"The REM and Nirvana successes don't mean much to me except as a potential distraction for bands who want to cash in on the trend. Don't try to sound like someone else. REM and Nirvana don't sound like anyone else".
Tom petty said "Nirvana to me was the most significant thing since the Beatles"