From the introduction:
Demiplanes are alternate dimensions of limited size, usually formed as an offshoot of another dimension. Many of these demiplanes will have unique properties that are not found on the prime material plane. A considerable number of these small demiplanes are created to serve as a place of safety for a wizard. However, from time to time other powerful entities will form planes of finite dimension to serve a specific purpose. Many created demiplanes can remain in existence long after the passing of their creators, waiting for explorers to discover their whereabouts. They may find an alien place whose very purpose is a mystery, or a safe refuge from the dangers of planar travel. Each such plane is linked to another plane or demiplane, and can be accessed from there by those who know it exists.