I think rock music is the most complex genre ever for being so wide. I mean, you can listen to Coldplay and then right after, listen to Foo Fighters. They're both rock bands, but they're so different that it's kind of hard to think of them being in the same genre and in the same rock world. Some say that the strong beat in 2 and 4 in a 4/4 time signature, is the defining characteristic of rock and roll, because without it, rock ceases to exist. But I'm sure I heard this beat in other genres, such as ska, country, jazz, R&B, pop, and even in some hip-hop songs. What defines rock and roll? Melody, harmony or rhythm? The way the guitar is played or the bassline?
Also, I didn't know that Avril Lavigne was a pop artist. I thought her music was classified as ROCK, as she performs songs with that cliché empashis on the backbeat.