The cosmos consist of four basic types of plane:  Material, Transitive, Elemental, and Outer.  The Material planes, such as Dalleth, are where the deities exert their influence and where Fate presides.  The transitive planes are The Plane of Shadow, the Ethereal Plane, and the Astral plane.  These planes all function as per the standard D&D Cosmology, with minor variances.  Elemental planes are likewise identical to the D&D planes.  The Outer Planes are not nearly as clear-cut, however, and do not follow the standard D&D paradigm.

Material Planes

Material planes are all separate, but none of them are coexistent with each other.  They can be thought of as layered, with no defined order to the layers.  That is, they all inhabit the same basic region of hyperspace. 

Ethereal Planes

Each material plane has it's own independent ethereal plane.  This plane functions just like the Ethereal plane described in the DMG.

The Plane of Shadow

Each material plane has it's own independent shadow plane.  This plane functions just like the Ethereal plane described in the DMG.

Outer Planes

More info coming soon!

Life in the Planes

Beings who live in the planes on a regular basis develop a number of strategies for maneuvering in such dynamic environments.

Movement between planes

There are a number of ways to move between planes, some easier than others. Most of them involve finding a path through the astral plane, although some methods are much faster and more reliable than others.

Astral Projection

Through magic or psionics, it is possible for individuals to venture into the astral plane while leaving their bodies behind.  They create astral bodies with silver cords as described in the spell Astral Projection.  Astrally projected travellers are still affected by the traits of the astral plane, despite not having a physical body.

Astral Travel

Astral travel differs from projection in that the travellers physically enter the astral plane, typically through a gate.  Travelling in this manner is otherwise similar to astral projection, except that the traveller is physically moving from plane to plane, and has no silver cord.  Death on the astral plane in this form is permanent.

Instantaneous Travel

Many magical and psionic powers allow travellers to journey directly between two planes.  In these cases, the path through the astral plane is traversed automatically, with no worries about finding the right destination plane.  Many planes cannot be entered this way, however.
A number of intra-planar travel techniques do this as well, including the Teleport and Dimension Door spells.  They only touch the astral plane very briefly, however.

Rifts

At many places, natural rifts occur between two planes.  These unstable rifts connect one plane directly to another, passing through a specific astral path.  However, material passing through these rifts rarely arrives intact unless gates are formed.  Any object passing through a planar rift has a 5% chance to come out on the other side intact.  If this check fails, a living being still has a 10% chance to come out still alive, and instead takes 10d10 points of damage.

Planar Gates

In ages past, the Illithid empire erected massive stone portals between planes.  Most of these gates are now dead, but a number still function.  There are even a few planar denizens with the magic necessary to erect new gates, although magical portals are not as reliable as the Illithids' psionic ones.

Planar gates can take almost any appearance, or even be entirely invisible.  However, the gates created by the Illithids are most commonly enormous stone rings, veined with crystal and carved with runes, that stand upright, the bottom section buried underground unless the gate floats in the air.  A swirling mist with subtle changing colours floats in the middle of the ring.  Intra-planar gates also exist, and are typically much easier to construct. 

Gates may also appear simply as large crystals pointing skyward.  These gates typically must be activated with a special mental command from a psionic creature (this takes 1 round and costs 1 power point per person or 200 lb being transported).

Additionally, the astral demiplane of Tor has thousands of stable portals connecting to almost every known plane, and many as yet undiscovered ones.  See Tor'il ^{(link missing)} for more information.

Astral Pools

The astral plane is home to thousands of swirling coloured pools that act as stable portals to the planes.  Some of these are one-way, dumping the traveller into either a predefined or random place on the destination plane.  Other pools are two-way, having a reciprocal gate at the destination.  The reciprocal gate is often an identical pool, especially on astral demiplanes, but it may also take another shape.  Many gates connecting to the astral plane in this way are actually stabilized rifts, built by the Illithid empire.

Adapting to change

Certain magic items have proven invaluable for less adept travellers, and as such are easier to find in astral ports.  In particular, rings of feather falling are employed for planes with subjective gravity, since they allow the wearer to control their movement much more easily.  When affected by feather fall, characters get +10 to their Wisdom check when changing direction.