A Plane is a set of space and time bounded by a set of restrictions.
Creating Planes
Spellcasters
Creating a plane can be simply for even apprentice mages. Spells such as Rope Trick, rely on temporary planes crafted entirely by the spell caster. Most planes created at this level can last between 1 minute and 1 day, but creating planes that sustain longer than that is an artwork mastered only by the gods and the most mythical spell casters. However, many spell casters have been able to take advantage of long dead planes as storage for items such as Bags of Holding. No notable planes created by mortals can be found as many disappear before gaining notoriety.
Gods
A deity of sufficient power is able to conjure up planes merely through divine force. Sometimes, the creation of a plane is a side-effect of creating a "Prime Material Plane", as these special types of planes are the seeds for an entire universe. These planes typically become permanent parts of the Great Wheel and are often created for a divine purpose, such as housing that deity's collection of souls, or for segregating creatures that the deity favors from barbarians in Quelmar.