Umbran's definitions are right on the mark, I think. Keep in mind that most of this is merely IMO. The only one about which there is no credible argument is Tolkein.
Great Literature: Tolkein, Moorcock, Lord Dunsany, Silverberg, LeGuin, Elizabeth A. Lynn.
I like, but are in no way 'Great Literature: Eddings, Martin, McCaffrey, Lackey (some of her stuff is horrid, some wonderful), Ru Emerson, Glen Cook.
Authors I think are shameless hacks: R. A. Salvatore, Terry Brooks, Robert Jordan, Terry Goodkind, and a whole host of others.
Edit: Added a few I forgot. Edit again: Got an author's name wrong!
