A plane is a dimension of existence. There are a range of planes, some of which are detailed below.

The Material Plane:

The "main" plane, the Material Plane is the home of most creatures. The various PC races are from the Material Plane and most of them spend their entire life there.

There are no special game effects for being on the Material Plane; the Material Plane is the default assumed by the rules. When in doubt, assume the Material Plane follows the same physical laws as the real world.

The Astral Plane:

The Astral Plane is the plane closest to the Material Plane. Spells cast to leave the material plane usually take the caster to the Astral Plane and vice versa. No creatures are native to the Astral Plane.

On the Astral Plane, there are points known as "planar windows". A planar window is tied to one particular plane and you can use magic to shift to that plane via the window.

While on the Astral Plane, gravity is subjective. You can, as a free action, change the direction gravity pulls you (no gravity is the default and can be chosen).

Elemental Planes:
The four Elemental Planes are the source of the fundamental parts of the universe. Air, Earth, Fire and Water; each provides one part of everything.

While on an Elemental Plane, gravity is subjective. You can, as a free action, change the direction gravity pulls you (no gravity is the default and can be chosen).

All elemental planes are infinitely large. However, whenever someone enters an elemental plane, they always appear in the same area.

Plane of Air:

By far the safest of the Elemental Planes, the Plane of Air is an endless void of air. The plane of air is the only elemental plane with breathable air on it naturally.

Plane of Earth:

The Plane of Earth is an endless maze of caves and caverns. To make navigation worse, the cave walls are slowly shifting around.

Plane of Fire:

The Plane of Fire consists almost entirely of endless fire. Creatures on this plane take 3d6 fire damage every round, unless the creature is in one of the plane's scattered safe areas. These safe areas are much cooler.

Plane of Water:

The Plane of Water is filled with water. However, the plane has no water pressure, making it easy enough to create an empty area.

Alignment Planes:
The three Alignment Planes rule over the spirit. Each tied to part of the alignment spectrum; Good, Neutral and Evil.

When a creature dies, its soul is sent to the Alignment Plane matching its alignment (souls in a plane are known as "patrons"). All alignment planes are infinitely large. Patrons do not need to eat, sleep, drink or breathe, but can do all these things if they wish.

Patrons can not die; any effect that would kill them instead destroys them for 1d6 hours, after which they reform at their point of death. Patrons can not leave their alignment plane unless they are brought back from the dead first.

Special Note: Divine casters go to the alignment plane where their god/goddess lives, even if their alignment isn't the same.

Realm of Light (good):

The Realm of Light is an infinitely large kingdom. A peaceful land, none need want here. Anything you could desire, at your fingertips. The lack of challenge can bore some, however.

Realm of Shadow (neutral):

The Realm of Shadow is an infinitely large castle. Large areas to see and explore.

Realm of Darkness (evil):

The Realm of Darkness is an infinitely large series of caves. At first glance, it looks like the plane of earth. Not a bad place; the company might be problematic, though.