Biome

Biomes in Minecraft are areas with specific height, light levels, vegetation, and types of blocks. There are 6 biomes currently in Pocket Edition: Desert, Tundra, Forest, Plains, Ocean, and Extreme Hills.

Desert
Block Types: Mostly sand, With Sandstone 3-4 Blocks below it. Gravel forms often in patches

Height: Sea level, occasionally small hills form.

Vegetation: Cacti, on rare occasions sugar cane forms near water.

Other Notable Features: Water is almost always found nearby deserts.

Sub-Biomes:

Beach: Smaller deserts bordering an ocean

Oasis: Small patches of sand around a small water source.

Tundra
Block Types: Dirt, usually with snow on the top. Water sources have ice on the top.

Height: Tends to be quite mountainous.

Vegetation: Pine trees can only be found in this biome. Grass underneath the snow.

Other Notable Features: (None yet, input wanted here)

Sub-Biomes: None

Forest
Block Types: Mainly Dirt and Grass, with the somewhat rare patch of gravel.

Height: Rather close to ground, but bumpy.

Vegetation: Oak and Birch trees, with sugarcane occasionally spawning near water. Mushrooms sometimes spawn underneath trees, where the light level is low enough.

Other Notable Features: Not safe to travel at night, because trees are very dense and can obstruct vision, but allow plenty of room for monsters to spawn. Creepers are especially a problem in this biome.

Sub-Biomes: Clearing: A small to medium sized area devoid of trees.

Plains
Block Types: Mostly Dirt and Grass, Gravel forms (Rarely) in patches.

Height: Mostly Flat, but small to medium sized hills tend to form.

Vegetation: Mushrooms and Flowers, the area is rather barren of trees.

Other Notable Features: Very flat and grassy, making it the most suitable location to build large structures, but rarely far from a forest.

Sub-Biomes: None.

Ocean
Block Types: Oceans are extremely large areas of water, sometimes extending for several hundred blocks in all directions, and can be as much as 75 blocks deep. The bottom consists of either dirt, sand, or gravel, and clay can sometimes be found near the shore.

Height: Sea level (Obviously), and can extend from 30-50 blocks deep, though can be up to 75 blocks deep

Vegetation: No vegetation underwater.

Other Notable Features: If you can create a solid surface to build on, is the absolute flattest biome in the game.

Extreme Hills
Block Types: Dirt, stone, and grass.

Height: Extremely hilly, mountains rise very high and are incredibly steep.

Vegetation: There is very little vegetation due to the lack of available space. Trees can sometimes spawn, and mushrooms spawn underneath overhangs.

Other Notable Features: This is the most unusual biome, overhangs tend to form and extend for quite a ways. Another common oddity is "Floating Islands", Where part of the terrain is suspended midair, with no apparent connection to any cliff or the ground. These are hard to access, but, if it is big enough, are among the most interesting places to build your home.

Sub-Biomes:

Hills: Smaller mountains which are very common in most biomes.