There is a distinct, several-layered structure to the campaign world.
The basic level is the Material plane. Here is found the world of Narth, the fourth planet orbiting a G-type star. All the worlds in its solar system are artificial, having been constructed by the metagod Asutine some ten thousand years ago. Asutine's motives for this act, and its subsequent creation of the gods, are unknown -- and possibly unknowable.
"Parallel" to and overlapping each other and the Material plane are the four "elemental" planes. These planes have a one-to-one correspondence with each other and the Material: every location in the Material plane has a counterpart in each of the elemental planes. Because of Narth's unique construction -- pure elements from their planes were blended with more mundane material when Asutine sculpted the planet -- it has a special affinity for the elemental planes, and their inhabitants may easily manifest on Narth. Normally such manifestation is caused by summoning magic, but certain sites on Narth -- such as volcanos or deep sea fissures -- are particularly close to one or more elements, allowing spontaneous or even unrestricted transfer from Narth to the appropriate plane.
Surrounding and permeating the Material and the Elementals are the Astral Plane and the Dream Realms. Where the former five are "physical", these two are "mental" spaces. The Astral is independent of the physical realms, and appears to be the matrix in which they formed. It has no native or "natural" material composition of its own, although "alien" matter can be imported to the plane, and will not suffer any (known) ill effects there. The Astral Plane also seems to have no native inhabitants or life forms. Mages and certain mystics have found ways of projecting their minds into the Astral.
However, everyone has projected their minds into the Dream Realms at one time or another. Like the Astral, this plane is "mental" in nature, but where the Astral exists independently of the physical world, this plane is created and maintained by the mental activity of sentient life, and in the absence of sentient life it would not exist. It's called the "Dream Realms" because it is shaped and populated by the subconscious and dreaming minds of the races of Narth; it is a surreal landscape of nightmares and fantasies, dangerous to the few willing to enter it physically via magic.
Because it is the product of living minds, it differs from the Astral in that it is entirely local to the planet. It is not possible to travel past the dreams of men and elves and find the dreams of alien creatures beyond the experience of mortals. Nor is it possible to find the dreams of the gods, for these are also elsewhere. The Dream Realms extend only a little further than the atmosphere of the planet.
However, the Dream Realms have no sharp "edge"; travel far enough, and one slowly enters the Astral Plane. Likewise, it is possible to enter the Dream Realms from the Astral, for the two overlap, probably because of their similar sensitivities to living minds. Access to both realms is similar in style and "location", and a careless traveler can reach the wrong one by mistake.
The overlap of the Dream Realms and the Astral Plane has one other effect, of great importance to many mages. A large portion of the Astral Plane is, because of the proximity of the Dream Realms, responsive to the will and desire of astral visitors, and can be shaped into matter and energy. More than a century ago, mages have discovered this "contaminated" area; in the ensuing decades they learned how to use its special properties and turned it into the vast magical construct known as Cerebrospace.
Besides the Astral and the Dream Realms, there is one more zone congruent with the Material Plane: Godsland. This is the "metaspace" which connects all the realms and planes of the gods; unlike the other realms and planes, it is less a universe than a place -- an endless flat plain of unceasing variety, in the center of which is the Hub of All. The gods use Godsland as a meeting place, and they have direct access to Narth from it, should they wish to manifest there in some way.
Finally, the many planes and demi-planes of the gods defy simple description. They are all but infinite in number, but none is as large as the Material Plane. In the Sidhas tales of the creation of the Narth, Asutine sculpted the gods from the very fabric of these many planes. The exact truth is uncertain, but it must be noted that besides the gods, only these planes' own inhabitants are concerned with them. They are not home to, nor do they hold the interest of, such metadivine beings such as the Watchers Above, Asutine and the Void. Strangely, the material world seems to be the chosen home to beings both lesser and vastly greater than the gods themselves; and the gods have long puzzled over this paradox.