Some Basic Concepts (PWH)

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Before learning the specifics, it's essential to grasp a few multiversal truths and get the basics of "who" and "where" straight. 
==Three Truths==
The multiverse operates around three basic principles. Learn these, and a basher'll have a head start on understanding how things really work.
The Center of the Multiverse: Primes think their little worlds sit at the center of it all, but it ain't true. Fact is, nothing sits at the center of the multiverse—how can there be a "center" of an infinite expanse? So, really, no place is more important than anywhere else. By the same token, wherever a body stands is the center of the multiverse—at least from his perspective. 
The Unity of Rings: The multiverse tends to form endless rings—whether physical or philosophical. Sigil, the Outlands, the Outer Planes, and even the Elemental Planes form rings without beginning or end. Follow any ring, and a body';; always end up where he started. 
Rule of Threes: Things happen in threes. Simple enough? It's not logical, but it's almost always true.
==Who's Who==
Plannars are natives of the Outer and Inner Planes—whether they're human, humanoid, or another shape altogether. Planars have a few special powers and restrictions that make them different from primes. First of all, they can see portals to other planes, as described in the "Traveling the Multiverse" chapter.
Second, it's crucial to know where a planar was born, because he's extraplanar everywhere else. This means that everywhere except on his home plane, a planar's subject to protection from evil, holy word, banishment, unwanted contact other plane communications, and any other such spells that specifically deal with extraplanar creatures.
Third, planars can be summoned without warning by spells or psionic abilities designed to grab planar creatures from the planes and haul them to the caster's location. Spells of this nature include monster summonings cast on planes adjacent to the planar's current position; or "summon planar creature," a psionic psychoportative science.
Primes, who 're often unfairly tagged "the Clueless," are simply natives of the Prime Material Plane. While they're often considered know-nothing leatherheads and easy marks for peelers, primes do have one big advantage over planars: They're not subject to any spells that summon, command, or banish extraplanar creatures, no matter where they go. For example, a good-aligned prime in the Abyss isn't subject to a protection from good spell, whereas his planar companion would be if that planar wasn't an Abyssal native. 'Course, any sod in the Abyss has more to worry about than that....
Petitioners are the spirits of deceased primes and planars who inhabit the plane of their deity or alignment. Petitioners take many shapes but are usually indistinguishable from other folks, at least at first. They don't remember their former lives and only vaguely reflect their previous personalities; they're shaped into whatever form their power or their alignment requires. About the only thing they have to work toward is the hope of attaining union with their power or plane. They never leave their home planes for fear of permanent dissolution. 
Proxies are the hand-picked servitors of the powers—those who some call gods. As servants of their deities, proxies act in their powers' interests and according to their wishes. Some proxies retain their original forms, while others are reshaped into forms favored by the powers. Naturally, their personalities and intentions vary, depending on the particular powers they serve. Regardless, it's a sure thing that proxies answer to their gods and only to their gods, and a basher had better treat 'em with the respect they're due. Not all berks have a direct pipeline to their deities, after all.
Powers—whether a body worships them as gods or just thinks of them as really mighty bloods—are the high-ups of the planes. They rule over certain areas, establishing their dominion and enforcing their whims as their natures demand. They offer spells and abilities to those who worship them. Within their realms, the powers rule supreme; no mortal cutter's ever going to come close to challenging their might. 
==What's Where==
Planes are infinite expanses that usually represent a certain alignment or element, and maintain their own physical laws, environment, and natives. The Outer Plane of Arborea, for example, exemplifies the qualities of chaos and goodness, and in general the plane's inhabitants exhibit characteristics of individuality and virtue. 
Layers are portions of a plane, so to speak, that are linked to the other layers of the plane yet maintain their own unique environments. To continue the example above, Arborea has the three layers of Olympus, a layer of green forests and mountains; Ossa, an endless shallow sea; and Pelion, a stark white desert. All three layers are "infinite" unto themselves, yet ¥e contained within the larger plane. 
Realms are smaller portions yet, occupying only a part of a layer and unified by specific conditions and natives. Usually, "realm" refers to an area in which a power resides, but realms aren't absolutely limited to powers. In Arborea's first layer of Olympus, both the Greek and the elven pantheons maintain their realms, which include the homes of the gods, the proxies and planars who live there, and all the petitioners from all the countless Prime worlds who belong there. The realms of Olympus (Greeks) and Arvandor (elves) appear just as infinite as the layer they rest in, yet much exists on the layer besides rowdy Greeks and fey elves.
Sites are smaller, very specific locales, often—but not always—within realms. Somewhere within the realm of Arvandor lies the Evergold, a pool dedicated to the goddesses of beauty, which grants a body matchless comeliness and charisma if she can find it. Outside of any realm but still in the layer of Olympus, the Gilded Hall of the Sensate faction rings with continual revelry.
Finally, there are Towns, which even the simplest addlecove understands. Like sites, towns can be located within or outside of realms; the treant-town of Grandfather Oak flourishes within Arvandor, while the philosophers of the town of Thrassos thrive just outside the realm of Olympus. The ultimate town, however, is a place from which all others can be reached: a place known by many names, but most often called Sigil.