Minecraft wiki entry for How_to_play, associated text: Starting Map: If this setting is turned on, every player starts with a map in their hotbar.
Bonus Chest: If this setting is turned on, a chest with some starting loot appears near your initial spawn location.
World Type: This setting controls the way the world is created. Select Infinite, Flat, or Old from the drop down menu to change the world type.
Infinite worlds go on and on forever.
Flat worlds do not have any hills or trees, just an infinite flat area to fill with your creations.
Old worlds limited in size and height.
Seed: This is a unique number for the world. By entering a seed when you make a new world, you can control how it is created. You can also use a word or phrase for the seed and the game converts it to a number for you.