I think All Music Guide has a good summary of what's in which genre. If you take a look at the sub-genre's of Rock that the AMG publishes AMG Rock Genres , there's one category - Art-Rock/Experimental, which includes the following sub genres:
* Prog-Rock/Art Rock
* Kraut Rock
* Noise-Rock
* Neo-Prog
* Experimental Rock
* Canterbury Scene
* Avant-Prog
AMG goes on to define each, and they give some interesting history.
* Prog-Rock/Art Rock
* Kraut Rock
* Noise-Rock
* Neo-Prog
* Experimental Rock
* Canterbury Scene
* Avant-Prog
AMG goes on to define each, and they give some interesting history.