Another Beer Advocate link: http://beeradvocate.com/beer/style

Those folks make a three-level hierarchy:

1. Yeast Type
2. Origin
3. Style

You can see another structure emerging within theirs, however, in which descriptors are tacked on to a style. For example, you have an ale, a pale ale, an imperial pale ale, a double imperial pale ale, an american imperial double pale ale, etc.