Minecraft wiki entry for History_of_world_generation, associated text: Island: World spawns as a large island surrounded with water, smaller island off the coast are comon too. Border block is water as well, sand will generate.
Floating: World generates in rings of floating islands high in the air. Border block is water. Sand is common although caves often lead into the void.
Flat: world is a perfectly flat plane with trees growing. Border block is grass. Sand does not generate.
Original: Resembles classic 0.30 terrain generation
Inland: