Plains is an extremely simple small map. Its main purpose is for people who do not want terrain/caves being a problem and want a completely flat and clean world. At around mid height, everything below is solid dirt. Nothing else. Everything above is clean sky. The cavern and underground wall layers have been removed, so as you dig down there will be nothing but sky. This allows for easy underground tunnels without having to dig out heaps of dirt wall.
This is the 'Plains' Template.