Oerth, the Material Plane for the D&D cosmology, consists of self-contained spherical bodies hanging in space. Oerth has all the usual traits and connections of a material plane. In connects to the Astral Plane, the Etheral Plane, and the Plane of Shadow, and travelers from Oerth may reach the Inner and Outer Planes through the Astral Plane.
Oerth's cosmology is laid out in the planner diagram presented to the right. A sphere of six Inner Planes surrounding Oerth, which also connects to the Astral Plane, and through it, to the Great Wheel of seventeen Outer Planes. Each of the six Inner Planes is not connected to another Inner Plane but each of the Outer Planes is connected to its neighbors. If you know where to look, you can find your way all the way around the Great Wheel from Celestia to the Abyss and back.
In addition there are more small demiplanes connected to Oerth than can be counted. Many of these have been created by deities or powerful wizards as places to hide their riches, get away from the mundane world, or imprison particularly unpleasant enemies.
Oerth does not have any kknown connections with Alternate Material Planes. These alternates may exist intermittently if you want your campaign to visit them.
While Oerth itself does not have elemental or energy traits, particular locations on the plane have pockets that have elemental traits. These areas are often located near portals or Vortices to the various Elemental Planes.