Planes: Infinate expanses that usually represent a certain alignment or element, and maintain their own physical laws, environment, and natives.
Layers: Portions of a plane, so to speak, that are linked to the other layers of the plane yet maintain their own unique environments. A layer is "infinite" unto itself, yet contained within the larger plane.
Realms: Even smaller portions yet, occupying only a part of a layer and unified by specific conditions and natives. Usually, "realm" refers to an area in which a power resides, but realms aren't absolutely limited to powers.
Sites: Smaller, very specific locales, often (but not always) within realms.
Towns: Like sites, towns can be located within or outside of realms.The ultimate town, however, is a place from which all others can be reached; a place known by many names, but most often called Sigil.