Overworld

The Overworld is the default dimension in Minecraft Pocket Edition. The Overworld consists of various biomes, flora and fauna, etc. There is a day/night cycle as well.

Mobs
There are many types of mobs that spawn in the overworld-

Passive

 * Pig
 * Cow
 * Sheep
 * Mooshroom
 * Chicken
 * Villager
 * Bat
 * Ocelots (0.12.1)
 * Snow Golem (0.12.1)

Hostile mobs

 * Zombie
 * Zombie Villager (0.12.1)
 * Skeleton
 * Creeper
 * Zombie Pigmen (Spawned from Nether Reactor)
 * Silverfish
 * Slimes

Neutral mobs

 * Wolf
 * Spider
 * Enderman
 * Iron Golem (0.12.1)

Landscape
The landscape in the Overworld consists of mainly grass blocks, along with a variety of blocks in various different biomes- Ores spawn underground that can be used to craft tools and armor; Old World types are surrounded by a wall of Invisible Bedrock; and generated structures like trees, dungeons, and villages also appear.

Trivia

 * The Overworld is the first and only Dimension for PE.
 * The Nether will be added in Update 0.12.1.
 * The Overworld is the only dimension which has daylight cycles in any of the Minecraft game platforms.