Originally Posted by tore

To me, at it's simplest, rock music means music played by a rock band which basically means played by someone doing something like electric guitar, a drum kit, bass guitar and perhaps someone on keyboard. If these guys play jazz, you call it jazz rock or fusion. If they are experimental in some way, you may call it symphonic rock or art rock or what have you.

That's the jist of it the way I see it .. (and again also why I'm hesitant to accept Rock Bottom as prog "rock")