All of Tony's suggestions are, I think, accurate, but you have to consider where these terms came from. ``Progressive'' seems to come from the early to mid seventies, when the pop music scene was inundated with stuff like ``Afternoon Delight'' and ``Chevy Van''. ``Alternative'' was coined in the late 80's when there was a preponderance of equally bland pop music. Both terms essentially came about as a backlash against what pop music had become, which was largely innocuous radio-friendly pap. Neither term has much meaning on its own, but has come to describe what the popular backlash music sounded like. In the seventies, it tended to be orchestral-influenced stuff. In the eighties, it was less easy to describe, but has as cohesive a sound. In the nineties, it was ``Grunge'', which ended up being just hard rock (there were probably only two or three actual grunge bands popular back then, but many more got that appellation). I expect we'll be seeing another name pop up pretty soon. If we're lucky, it's not going to just be ``New'' rock, in the form of Limp Bizkit, etc.

BTW, CDnow keeps a reasonable description of music genres. Click on the genre links to the left on their page and click a more specific subgenre to get a description. They seem to all be taken from AMG, but here's the one for prog/art rock.