Planes are, collectively, the various dimensions of existence. Kishar is on the Material Plane, the plane of mortals, but there are innumerable others as well.
Overview
Planes vary greatly in their traits. Some are effectively infinite, while others are quite bounded. Gravity, light, and the passage of time may vary between planes, amongst other fundamental factors.
To the inhabitants of Kishar, the planes are generally poorly understood - even the most learned and capable caster rarely ventures into the unknown of the outer planes, so great is the potential danger. Travel to the planes is largely done through naturally occurring portals, coterminalities, and Warps, rather than through the casting of spells, as doing so adds extra risk and unpredictability.
Planes
The following is an incomplete list of planes:
Demiplanes
Unlike full planes, new demiplanes can be brought into being by strong enough casters, and therefore are even less well-known. The following is an incomplete list of demiplanes: