Normal: The default map theme. The map's contents would be based off the level type option. The weather was partly cloudy and trees and flowers would generate
Hell: Features significantly less lighting during the day. All surface water is replaced with lava. Mushrooms are abundant on the surface.[verify] Surrounding water also becomes lava.
↑"I've noticed that fog color is now based on how dark your surroundings are. I didn't notice this in the earlier version, so I'm assuming this is new. It makes a cool eyes adjusting to the light effect when entering and leaving caves." by MCFUser1359 – Minecraft Forum, January 7, 2010.