Mid-World is generally divided up into three sections: In-World, Mid-World, and End-World. It is the geographic center of All-World.
Before the world "moved on", In-World was the center of civilization. Mid-World (the section) contains such lands as the Waste Lands. The line between Mid-World and End-World is formed by the arc of the Callas. End-World is the location of the Dark Tower itself. It is also noted there is an oil refinery somewhere here.