The best discussion I've seen so far on this topic is in Steve Long's _Fantasy HERO_, the sourcebook for the HERO System. I've got the 5th Ed version of this tome, and it really _is_ a tome; 400+ pages of densely spaced type packed with all sorts of information. A lot of it is crunch for HERO, naturally, but the discussion on fantasy subgenres translates to any ruleset you're using.
Crossworlds fantasy: stories where the PCs are real-world people who get transported into a fantasy world. Think CS Lewis' Narnia.
Epic fantasy: stories featuring grand, romantic, monumental struggles against a vast, overwhelmingly powerful foe. Larger-than-life tales, told with a totally straight bat. LOTR, the Belgariad, the Dragonlance Chronicles, etc.
High fantasy: Epic fantasy by the rest of us.

Dungeons, dragons, weird and powerful magic, but with rather more down-to-earth/cynical/humorous characters than are usually found in epic fantasy. Ursula LeGuin's Earthsea, Vance's Dying Earth, Steven Brust.
Low fantasy: "Realistic" fantasy, ie little or no magic, low-powered characters, often a very pragmatic approach to things. The Three Musketeers and other "swashbuckling" tales, GRR Martin's Song of Fire and Ice.
Swords and sorcery: Conan. Culture is for pansies, the good guys are all barbarians. "Good" being relative, since everybody is generally looking out for number one.
Urban fantasy: Modern fantasy, ie magic in the 20th/21st century. Buffy, White Wolf's World of Darkness, Shadowrun, Urban Arcana, etc.