The World Axis Edit
The basic model by which The Planes' relation to one another can be described. They are a sort of vertical column, with the Astral Plane at the top, followed by the Material Plane, the Elemental Chaos Plane, and the Abyss at the very bottom.
The Astral Plane Edit
Celestials dwell here. These are self-appointed paragons of goodness, though what exactly constitutes 'goodness' is something they dictate, and woe be unto those who disagree with them. Most gods considered 'good' in the mortal realm reside here, as do other celestials.
Once, long ago, all gods lived here, as did demons, devils, and elemental creatures. But the Astral Gods decided they did not like sharing the realm with creatures and gods they considered distasteful, so they attacked the Elemental Chaos gods and their ilk and cast them out, forcing them to reside in a new plane below the mortal realm.
Celestials consider any being that is from the Elemental Chaos plane to be evil and take a 'kill-on-sight' approach toward them, regardless of the being's actual moral alignment.
All mortal souls, once they have escaped the Feywild or Shadowfell, end up here. But only those that the Astral Gods deem 'worthy' may remain, while all others are banished to the Elemental Chaos plane.
The Material Plane Edit
The main portion of the material plane is the world in which most mortals live. There are also the Feywild and Shadowfell realms, which are separate, smaller planes that mirror parts of the Material Plane and touch against it in some places.
The Feywild Edit
A land of happiness, where mortals that die happy and fulfilled end up in their journey to the afterlife. Elves and gnomes originated here, as did other species and monsters. Not every species that lives here is benign, but they are all fun-loving and mischievous, and generally happy. Many creatures that have their origins here or reside here are considered attractive and/or cute by most mortal standards.
The Shadowfell Edit
A land of sadness and despair, where mortals that die sad and unfulfilled end up in their journey to the afterlife. Orcs and goblins originated here, as did other species and monsters. Not every species that lives here is evil, but they all carry a great sadness within them and a desire for more than life has bestowed upon them. Many creatures that have their origins here or reside here are considered unattractive and/or gross by most mortal standards.
The Elemental Chaos Plane Edit
The plane of Elemental Chaos is not quite so chaotic as its name would imply. It is divided unevenly into different regions, one for each element, in addition to the Demon Realm and the Devil Realm.
The gods that the Astral Plane gods considered distasteful were banished to this realm, and aside from caring for the elementals, devils, and demons that reside in the plane along with them, most gods that live in this realm devote their time toward planning an eventual counterattack to take the Astral Plane back for all non-Material Plane beings to live in.
Mortal souls that are banished from the Astral Plane end up here, where they remain forevermore, unless they are consumed by a devil or demon, or summoned back to the Material Plane. Consumed souls still exist, and once the demon or devil that consumed them is killed, the souls are free to wander the realm again.
Demons, devils, and elementals that are killed, whether in the Material Plane, Astral Plane, or Elemental Chaos plane, reform in the Elemental Chaos plane within a week.
There are small portals called 'windows' scattered throughout this plane that provide a look into the Material Plane. They can be 'tuned' through magic to look through any particular 'window' in the Material Plane. Denizens of the EC plane gather around these windows sometimes when particularly interesting things seem to be happening in the Material Plane.
The Air Realm Edit
The Air Realm is an open expanse with constant winds of varying strength. Here and there, chunks of earth drift in the openness. These earth motes serve as homes for the creatures of elemental air, and many motes are covered with lush vegetation. Other creatures live on cloud banks infused with enough magic to become solid surfaces, strong enough to support towns and castles.
Drifting cloud banks can obscure visibility in any direction in the plane. Storms are frequent, mostly on par with a strong thunderstorm, but occasionally more like fierce tornadoes or mighty hurricanes. The air is mild when it is not storming.
Hot, dry winds scour the earth motes in some areas, turning them into dry and barren chunks of rock. In other areas, endless storm fronts rage - walls of flames, smoke, and ash. This thick ash obscures sight beyond a few dozen feet, and the battering winds make travel difficult. In still other areas, frigid winds and snowstorms prevail. Earth motes in this area are covered with snow and ice.
The Earth Realm Edit
The Earth Realm is mostly made up of a chain of mountains rising higher than any mountain range in the Material Plane. It has no sun of its own, and no air surrounds the peaks of its highest mountains. Most visitors to the plane arrive by way of caves and caverns that honeycomb the mountains.
In large caverns beneath the mountains are vast cities where every building and significant object is made from precious stones and metals. Other mountains in the realm have lava seeping through their caverns, the air in and around them reeking of sulfur. In still other areas of the realm, horrid swamps with twisted, gnarled trees and thick, stinging vines grow from dense muck and slime. Here and there within the swamp, stagnant lakes and pools play host to thickets of weeds and monstrous swarms of mosquitoes. The few settlements here consist of wooden structures suspended above the muck. Most are built on platforms between trees, but a few stand on stilts driven deep into the muck. No solid earth underlies the mud of the swamp, so houses built on poles eventually sink down into it.
The Fire Realm Edit
A blazing sun hangs at the zenith of a golden sky above the Realm of Fire, waxing and waning on a 24-hour cycle. It ranges from white hot at noon to deep red at midnight, so the darkest hours of the plane display a deep red twilight. At noon, the light is nearly blinding.
The weather on the plane is marked by fierce winds and thick ash. Although the air is breathable, creatures not native to the plane must cover their mouths and eyes to avoid stinging cinders. The wind is always at least blustery, and rises to hurricane force during the worst storms.
The heat in the Realm of Fire is comparable to a hot desert on the Material Plane, and poses a similar threat to travelers. The deeper one goes into the plane, the rarer water becomes. Beyond a point, the plane holds no sources of water, so travelers must carry their own supplies or produce water by magic.
One portion of the Realm of Fire is covered in a great expanse of black cinders and embers crossed by rivers of lava. A great range of volcanic mountains with rocky peaks curls around this expanse. On the other side, lava flows through the volcanoes and pools into a great lava sea, with islands of obsidian and basalt jutting up through it haphazardly.
The Water Realm Edit
A warm sun arcs across the sky of the Plane of Water, seeming to rise and set from within the water at the visible edge of the horizon. Several times a day, however, the sky clouds over and releases a deluge of rain, often accompanied by spectacular shows of lightning, before clearing up again. At night, a glittering array of stars and auroras bedecks the sky.
The Water Realm is an endless sea, dotted here and there with atolls and islands that rise up from enormous coral reefs that seem to stretch forever into the depths. The storms that move across the sea sometimes create temporary portals to the Material Plane and draw ships into the Water Realm. Surviving vessels from countless worlds and navies ply these waters with little hope of ever returning home.
The weather in this realm is a lesson in extremes. If the sea isn't calm, it is battered by storms. On rare occasions, a tremor in the realm firmament sends a rogue wave sweeping across the realm, swamping entire islands and driving ships down to the reefs.
Life flourishes in the upper reaches of the realm. The deeper extents, however, where no sunlight reaches, houses horrible monsters and a cold, crushing pressure. A few islands dot the surface of the water, but few beings reside on them. Some areas of the realm are thick with soil and sludge, while others are frigid and choked with icebergs and sheet ice.
The Demon Realm Edit
The primary rule of law here is that there are no rules, for better or worse. Demons are inherently chaotic creatures, and so too is the realm, where one may find a kind, helpful being and a mean, malicious one living peacefully together, or two good-natured beings at war with one another. The realm's land is constantly changing, so that when you step into a building in one area, when you step back out you might be somewhere radically different. For this reason, it is unwise to split up while in this realm. To expect any particular thing while here is to be disappointed.
The Devil Realm Edit
The primary rule of law here is order. Every being that dwells here is inherently lawful, and does not abide those that refuse to acknowledge the law of the land. The realm is ruled by an elected leader whose reign lasts 100 years before the next ruler is elected. Kind and evil beings live side-by-side in this land, respecting one another's committal to lawfulness, even if they disapprove of each other morally. There have been good rulers of the realm and evil ones, but they have always been respected by devils of all alignments.
The Abyss Edit
A hole in the fabric of reality that resides directly below the Elemental Chaos plane, and which can be accessed from certain parts of that plane, where the two planes touch. Those that fall into this plane eventually fade away into nothingness, and no species or being can truly survive or thrive here.
The only way to truly destroy any being, be it a mortal soul, demon, devil, elemental, celestial, or even a god, is to banish them to the Abyss, and prevent them from escaping it. A being or spirit that fades away in this realm is truly lost from all planes of existence, no matter how powerful they were before. It is only a matter of time. The longer one stays here the less able they are to summon the willpower to try to escape.