The Mining World is a different dimension where you can go mine materials freely, without having to worry about affecting the terrain or areas near your home (or others' homes). Occasionally the world gets regenerated (and everything in the mining world is lost), giving the world fresh terrain and resources. (Regenerations will be announced at least a week before.)
Other than being separate dimension, the rate of ore in the Mining World is the same as the Overworld.
Visiting the world can be done by holding a Enderpearl in a Nether Portal. It does not cost you an Enderpearl to go to the mining world, but you may need the pearl to return (see Teleporting).
Don't die. If you do happen to die, your stuff will appear at the main world's spawn and you will appear back in the main world.
SKCraft