Terminology
A realm, often improperly referred to as a plane, is any discrete reality with its own definable properties.
Among the realms, there are several different groupings. The inner realms refer to the Mortal Realm, the Prime, and the Nether; the upper realms refer the Aether and the Astral; and the the lower realms refer to the Maelstrom and the Pit.
An imbricated realm is a realm that overlaps another realm, with features that closely resemble the realm it overlaps. The Prime and the Nether are imbricated upon the Mortal Realm.
A domain is any portion of a realm with its own unique properties. Some domains are created, and subsequently ruled over, by powerful beings (e.g., Immortals). Others occur spontaneously.
A span is a naturally-occurring passageway between realms. As a natural phenomenon, the formation or dissolution of a span can be predicted by someone with the right knowledge, although only about as well as you might forecast the weather on the Mortal Realm.
A gate is an artificially-created passageway between realms. Most gates are temporary creations of magic, although some rare creatures are also known to have the ability to create gates.
A spanner is any person who makes a living traveling between realms.
The Inner Realms
The Mortal Realm
The Mortal Realm, also called the Material or Corporeal Realm, is a middle ground. It shares foundational connections with most of the other realms in existence. For example, it includes all four basic elements of the Maelstrom, has imbricated Primes and Nethers, and its denizens project into the Astral through dreams. Likely because of how it is so ideally situated, beings from other realms often try to stake a claim. It is, for better or worse, a focal point for all other realms.
Shard Realms
Until about two centuries ago, the Mortal Realm was a single, contiguous realm. After the Shattering, however, it is better defined as many smaller realms connected by spans and separated by the Void. The relatively new term "Shard Realm" has been coined to describe these disparate pieces of the Mortal Realm. Shard realms are unique in that they have finite boundaries that can be easily crossed.
The Prime
Sometimes called the Fount or Wellspring, the Prime is a verdant realm of life, creation, and abundance. Areas of the Prime are said to be "imbricated" upon the Mortal Realm, which is to say that they overlap imperfectly. The Prime exists as a realm apart, but the pieces that overlap the Mortal Realm appear as distorted representations of the area that they overlap. In the case of the Prime, this distortion is one of light and life. An ordinary garden in the Mortal Realm may have a counterpart in the Prime that is filled with sunlight and overflowing with plant growth. The healing magic of disciples, and, to a lesser extent, druids, draws upon the power of the Prime.
There exists an area of the Prime that has no Mortal equivalent. Scholars disagree whether or not this "Deep Prime" is just a part of the Prime or another realm entirely. Many travelers who return from the Deep Prime describe an ocean of golden waters beneath a sky that never darkens. Most often, such travelers are changed by the experience, claiming to have touched upon the primal source of life itself. Others fail to return at all, or end up in other realms, suggesting the existence of naturally-occurring spans within the Deep Prime.
The Nether
In contrast to the light and life of the Prime, the Nether is a realm of shadow and death. The Nether overlaps the Mortal Realm in the same fashion, with imbricated areas appearing as desolate, decaying versions of their Mortal counterparts. The Nether is considered one of the more dangerous realms to travel because it is home to many forms of undead. Uordeks owe their racial traits to an inborn connection to the Nether. Likewise, warlocks who make pacts with undead patrons draw their magic from this "Nether World."
Just like the Deep Prime, there is also a Deep Nether that exists completely separate from the Mortal Realm. What brave souls have ventured there speak of an ashen desert where the souls of the dead drift soundlessly toward their final rest. Much like the Deep Prime, the evidence strongly suggests the existence of naturally-occurring spans within the Deep Nether, along which these spirits travel to other realms. However, none who venture far enough to learn the truth ever return.
The Upper Realms
The Aether
The Aether is a celestial realm, where travelers float weightlessly through a vast, tranquil sky filled with stars, comets, and other cosmic phenomena. In truth, each star in the Aether is its own domain, some created by Immortals to house loyal souls, some, like Beryl, forged by mortal magic, and many others whose owners have long since turned to dust. The spells of wizards draw upon a connection to the Aether, as do the racial traits of the empyrean race.
The Astral
Beyond the uppermost reaches of the Aether lies the Astral, the great sea of dreams. It is a realm shrouded in mist, where thoughts have substance and dreams take on a life of their own. The nature of the Astral makes it impossible to reach physically; travelers can only reach the Astral by projecting there mentally.
The Lower Realms
The Maelstrom
Bordering on the Mortal Realm, the Maelstrom is a realm of primordial chaos made up of roiling, clashing elemental forces. The landscape is riddled with volcanic flows and glacial drifts, and the sky is filled with tornadoes and storm clouds. Vast lakes dot the land, alternately boiling or freezing over. Worse still, the constantly shifting terrain makes the realm impossible to navigate. Sorcerers tap into the destructive elements of the Maelstrom to perform their magic known as Tempestry.
The Pit
Much like the Astral, the Pit is completely disjoined from the Mortal Realm, and thankfully so. This realm of fiery torment plays host to fiends and corrupt souls. Some scholars believe the Pit was created by the Ageless Ones to contain the evils of the realms, while others believe it was a realm from Beyond that long ago became enmeshed with our own. Either of these theories explains why most denizens of the Pit are so abhorrent and harbor such a innate hatred for the other realms of existence. Warlocks who make pacts with fiendish patrons draw their magic from the Pit.
Other Realms
The Void
Most people living on a Shard Realm, and all who travel between them, are familiar with the Void. At its most basic, the Void is the empty space between realms. Many believe that exposure to the Void has a maddening effect, appearing to onlookers as a churning nothingness. However, scholars contend that this is more a consequence of the mortal mind's inability to comprehend true nothingness than a property of the Void itself; indeed, the Void has no properties to speak of. Nonetheless, travel through the Void is perilous. Anything lost to the Void is doomed to fall forever through the bottomless abyss.
The Beyond
The horizon of reality extends infinitely farther than any mortal mind can comprehend. That which is "Beyond" refers to speculative realms far removed from those known. Such realms are so remote and alien, however, that they're probably best left unknown.
Life and Death
When one studies the realms, one can't help but confront questions of life and death. In many ways, the process of life touches upon every realm. Newborn spirits spring from the Prime and manifest within physical bodies on the Mortal Realm. Living bodies are composed of the base elements of the Maelstrom, while living minds are the stuff of the Astral. When a living being dies, its spirit passes into the Nether, where it may linger as an undead or move on to its final rest in an Aetheric domain or the Pit. A particularly stalwart spirit may be able to manifest a new body for itself, although even the hardiest cannot stave off final death forever. Only the Immortals have achieved this feat, and the secret to doing so has long been barred from mortal knowledge.