Its been my opinion that there are only three genres of music:
Classical, Jazz, and Pop.
For some reason it is easier to lump artists in the first two genres into their respective category and sleep soundly at night knowing that Franz Schubert and Iannis Xenakis are both Classical composers, and Charlie Parker and Ornette Colemen are both Jazz sax players.
On the other hand, when I make the argument that all pop is simply Pop, I've run into some static. So let me explain myself.
Slayer and Johnny Cash are more similar than dissimilar. Jason Mraz and Mushaga should be friends, and AC/DC and Dashboard Confessional fans should make nice and sit quietly on the bus ride home from school.