The planes of existence vary by nature. Some are vast, with numerous features and an endless amount of variety, such as the Prime Material Plane, while others, such as the Underworld, remain small and fairly homogenous.
Regardless of size and nature, all planes are self-contained universes that do not exist relative to one another within the traditional three dimensions. All planes have their own sets of immutable physical laws, though the nature of these laws may vary from plane to plane. Collectively, the planes constitute the multiverse. The multiverse's counterpart, the realm of chaos, lacks the orderly structure of a set planar layout, though some would consider it a massive plane in its own right (not to be confused with the Chaos Plane, which is a part of the multiverse).
See Category:Planes for a list of the planes known to mortalkind.