Cosmology
The most commonly accepted theories of the planes of existence state that the Astral Plane holds all the planes together. At certain locations in the Astral Plane the so-called cosmic forces or cardinal points can be found - these (or so it is thought) embody some of the fundamental concepts of reality. As far it is known, it is impossible for living beings to enter these points and return, for the nature of these forces would instantly overwhelm even the strongest magical protections and consume the unfortunate creature utterly. Ten cosmic forces are known to exist: Fire, Water, Earth, Air, Good, Evil, Law, Chaos, Positive Energy (also known as Life), and Negative Energy (also known as Death or Undeath). Each force always has a force in opposition to it: Fire versus Water, Law versus Chaos, and so on.
These forces exert a "pull" on large regions of astral space, similar to the pull a sun exerts on its planets, or a whirlpool on ships. Planes of varying sizes can exists inside of this region, but the "closer" the plane in question is relative to the cosmic force, the smaller and more unstable it will be. However, none of these planes is truly stable (with the exceptions described below), and all will break up into smaller planes eventually. Similarly, small planes will form and grow in size as they absorb other planes within the same region. The planes orbiting the cardinal points of Fire, Water, Earth and Air are often collectively referred to as the Elemental Planes. Each of these planes is "surrounded" by its own Ethereal Plane, which some sages speculate serves as a protective layer for the plane itself.
It should noted that terms like "close" and "far" aren't to be taken too literally when applied to the Astral Plane. While a visitor to the Astral sees it as he would see three-dimensional space, this is an illusion created by his mind - the Astral Plane does not have the same dimensions as the Material Plane does. Rather, "close" and "far" should be taken as metaphors - as a gauge of the affinity any given plane has for the cosmic forces. While planes orbiting a specific cosmic force usually do have a three-dimensional geography, they do not have any regions that are closer to the cosmic force than others - rather, the affinity of the plane for the cosmic force is equal everywhere. While certain magics or the force of will of certain creatures can make a part of such a plain hotter or more evil than others, this is only the result of individual meddling, and does not alter the plane itself in a fundamental way.
There are certain locations in astral space where the pull of two cosmic forces is equal, but in opposite directions. Most of these locations are unstable, which means that any plane at that location would soon be pulled in one direction or the other. However, there is always one (and only one) location between two cosmic forces that aren't opposed to each other which is stable. At these locations, large and stable planes form that contain elements of both of the cosmic forces, but are ultimately dominated by neither. These are practically worlds in their own right, and are host to a variety of creatures. For example, the devils hail from a plane balanced between the cosmic forces of Law and Evil, while the demons breed on a plane that hangs between Evil and Chaos. There are 40 such planes, one for each pairing of non-opposed cardinal points.
Other stable planes exist as well. These are mostly the realms of various deities, who can prevent their dissolution through their godly willpower alone. The realms of weaker deities can often be found in the vicinity of one or several of the cosmic forces, which allows them to siphon off some of their energy, but having a realm in such a location would be too limiting for the more powerful deities, and consequently their private planes are far away from any of the cardinal points. This might help to explain the general tendency towards neutrality among the most potent gods. Some sages speculate that these gods might actually attempt to transform their portfolio and themselves into an entirely new cosmic force. While this hasn't happened in recorded history, it doesn't mean that this cannot happen...
It was thought that the Material Plane exists at the "center" of existence and is "equidistant" to all the cardinal points, but new research suggests that this is true only for Rothea. It seems that other planets have their own Astral Planes (reachable from Rothea only through Epic Spells), and their own unique sets of cardinal points. The world of Yethrod, for example, seems to lack a cardinal point of Fire, but has an additional cosmic force of Sound. This revelation threw earlier theories in question that suggested that the cosmic forces were the wellspring of all creation, and made many scholars wonder how necessary the "cosmic forces" are for the actual cosmos.
Research is continuing to this day.
Planar Locations
The planes are vast, and more than one lifetime can be spent on exploring them without seeing them all. But terrible dangers exist next to heart-rending beauty...The City of Bells: Also known as the Plane of Evil Air, this dimension is filled with towers whose spires seemingly stretch to infinity - no matter how high one rises, the towers rise with him, and no matter to what depths an explorer plunges, he doesn't reach the base of these constructs. Yet this plane is not truly infinite, for after a time the buildings seem to repeat themselves - though only the natives are able to navigate with ease, and outsiders usually go mad when trying to discern this plane's geography. A pale mist that hangs everywhere and obscures long-distance vision reinforces the confusing layout of this plane.
This city gets its name from the myriad bells and wind chimes that adorn the towers. All of them constantly move in the strong winds, and produce a constant barrage of dissonant noise. Outsiders find these sounds disturbing to the extreme, but the natives that flutter between the towers do not seem to mind.
Planar Oddities
There are some locations that just don't fit into the "standard" cosmological theories, and thus provide a steady source for arguments between scholars of the planes. But whatever their ultimate classification, most of these scholars agree that they do exist.The Court of Elements: Most large stable planes that aren't the realm of a deity or other equally powerful entity are suspended between two cardinal points. The Court of Elements is large, and seems to be stable enough, but instead orbits between four different cardinal points - namely, those of Fire, Earth, Water, and Air. It spends about three year in the vincity of a particular cardinal point (commonly referred to as an "elemental season" by druids - currently the Court is in the third year of the Air season) before it moves on to the next one. The plane takes on characteristics of the cardinal point it is currently closest to - when in the Fire season, it appears as a parched desert, as a gigantic mountain range when in Earth season, as a vast ocean when in Water season, and as a vast collection of cliffs and canyons when in Air season.
The Court of Elements serves as an afterlife for the followers of the druidic faith. The protean nature of the land discourages permanent settlements, so these spirits take up a nomadic life and are well supported by the bounties of nature. Wildlife is abundant, and mostly appropriate to the season.
Some scholars speculate that there is a plane that orbits around the cardinal points of Law, Good, Chaos, and Evil in a similar fashion, but if it exists, no one has found it yet and talked of it.
Faerie: Home of the Fair Folk, Faerie goes by many names in many cultures. It has many portals to the material world, called trods, which take many forms - ancient monoliths, circles of mushrooms, strange caves with glowing fungi - and which can be used by the Fair Folk and those who follow them or are invited by them. Physically, it usually resembles the material plane on the other side of the closest trod, though everything seems to be on a grander scale and either more beautiful and more terrifying. Buildings created by the local Fair Folk likewise seem to be more impressive than their material world counterparts, though they are usually several centuries out of style.
Scholars suspect that the reason for this is that neither the Fair Folk nor Faerie itself can truly create something new. Instead they must ape other worlds and other planes, and likewise their customs reflect those they have observed on other worlds. Mortals who attract their attention should beware, for they are capricious and likely to involve them in games and intrigues that often have dangerous consequences without explaining them the rules.
The Road to Nowhere: This seems to be the afterlife for all those thinking beings whose souls aren't claimed by one deity or another after their deaths. These souls appear on a long, winding road in a body that looked much like it did during their life at some point, and they still remember everything they learned before their demise. This road winds through canyons, deserts, fertile plains, mountains, forests, and badlands, but it seems to go on and on forever. While it is possible to leave the road, it doesn't really get you anywhere, since this plane seems to loop back to itself - once you have traveled about 10 miles away from it, you will find the road again at pretty much the same place you started.
It is possible for planar travelers to visit the Road to Nowhere with the usual spells, but its inhabitants - the souls of the dead - are trapped there. The only way for them to leave is to be resurrected or - as rumors claim - to travel to the end of the Road, where everyone can allegedly find his heart's desire. The Road seems to go on and on forever, and nobody has ever been found who has claimed to reach its end, but yet the rumors persist.
Caravans and lone travelers constantly move along the endless landscape. It is dotted on both sides by small communities of people who have given up their travels, though most eventually hit the road again. For while countless people must have arrived here, somehow the population doesn't seem to increase - and this, in the minds of many, means that it is possible to reach the end of the Road...
The Underworld: Coexistent to the ethereal plane, the Underworld is the home of ghosts - especially the home of ghosts who worship the goddess Cryelis. It does not exist in all the same places as the ethereal plane - only close to the location where sapient beings have been sacrificed to Cryelis in rites known to her followers. Thanks to the activities of the Cult of Cryelis, most cities in the Parginian Rim and the Lake of Dreams regions have large local Underworld fragments, and now the cultists are trying to connect these isolated islands with each other. However, thanks to modern fast transportation and the population explosion of the last few centuries (which increased the pool of sacrifices dramatically), the Cult has established Underworld fragments in most cities.
Ghosts can cross over between the Underworld and the ethereal plane as a move action, but other creatures need to use a plane shift spell to get there. This makes combating ghosts there difficult, as they can simply move to the ethereal to avoid a fight. Physically, the Underworld resembles the material plane, though everything has a slightly dark tint to it. Whenever the Underworld is expanded through a sacrifice, the new area in the Underworld will have exact copies of all objects in the material plane at the equivalent locations (though without any material properties). Since many of the older Underworld fragments have expanded over the course of centuries, if not millenia, the styles of objects and buildings will vary dramatically. Furthermore, cultists of Cryelis know how to "sacrifice" objects so that their forms will appear in the Underworld (which they do frequently to gain the favor of ghosts), and all grave goods of a body buried in an area with an active Underworld fragment will be replicated there as well.
There are hints that Underworld fragments will gradually vanish over very long time spans if they are not attended by cultists, but so far, no one has figured out a reliable way of actively destroying or diminuishing them.