Pop music- n. Music that is, as the name suggests, popular among music listeners. Pop(ular) music can be anything really, dance, hip hop, rock, etc. Pop music also varies from country to country, and can be considered to vary even in different areas (what's popular in Midland, Texas may not be as popular in London, for instance...heck, they may even be the same sometimes, who knows). Popular music is not really a genre, but rather any kind of music that is spinning round and round the minds of consumers.
Elvis was popular in his time, so were the Beatles, Nirvana, COUNTLESS folk acts, and well, you get the idea. Popular music is just a term used to designate whats hot on the pulse of music. Just because a lot of people like something doesn't make it bad (not to say that I don't dislike some, if not a lot of pop music personally) because heck, maybe the collective consciousness can come up with some good ideas sometimes, eh? If you just listen to a band because it is not well known, or refuse to listen to a band because it is popular (God forbid you do both), then you are listening to music for all the wrong reasons, and should look into joining a secret club to satisfy your superficial desires to be "cool" because the cool folks just do whatever the heck they want, we dont care what the critics say, even if its good. Thats how cool we are. ;)