The cosmos teems with a multitude of worlds as well as myriad alternate dimensions of reality, called the planes of existence. It encompasses every world where GMs run their Adventures, all within the relatively mundane realm of the Material Plane.
The Inner Planes are elemental embodiments, the raw stuff
from which the rest of the rest of the multiverse is awesomely made.
Each Inner Plane is made up mainly of a single type
of energy that overwhelms all others. The natives
of a particular Inner Plane are made of the same energy or
element as the plane itself. The six Inner Planes are the
Elemental Plane of Air, the Elemental Plane of Earth, the
Elemental Plane of Fire, the Elemental Plane of Water, the
Negative Energy Plane, and the Positive Energy Plane.
These planes surround and enfold the Prime plane. At their innermost edges, where they are closest to the Material Plane (in a conceptual if not a literal geographical sense), the four Elemental Planes resemble a world in the Material Plane. The four elements mingle together as they do in the Material Plane, forming land, sea, and sky. Farther from the Material Plane, though, the Elemental Planes are both alien and hostile. Here, the elements exist in their purest form—great expanses of solid earth, blazing fire, crystal–clear water, and unsullied air. These regions are little–known, so when discussing the Plane of Fire, for example, a speaker usually means just the border region. At the farthest extents of the Inner Planes, the pure elements dissolve and bleed together into an unending tumult of clashing energies and colliding substance, the Elemental Chaos.