Overworld

The Overworld is the base dimension in Minecraft, and is presently the only dimension in Pocket Edition besides the Void. It consists of many different Biomes, and trees, mushrooms, Sugar cane,and Wheat all come from this dimension.

Mobs
Pretty much every mob the game has to offer is from here, besides Zombie Pigmen, which originate from the Nether Machine. Cows, Sheep, Chickens, and Pigs make up the passive fauna, while Zombies, Skeletons, Spiders, and Creepers act as hostile monsters.

Survival
Surviving here is pretty straightforward, the first thing you must do is collect wood from trees, so that you can make a Crafting Table, which is required to craft most of the items in the game. Then you should make a few tools, preferable a Pickaxe to collect stone, a Shovel to dig dirt, sand, and gravel, and an axe to collect wood. Then you should make a shelter, which can be done in two ways. You can make a simple house out of wood planks or dirt, or you can hollow out a cave in the side of a mountain or hill. Afterwards, you should dig a mine to collect some stone to upgrade to stone tools.