Great Wheel Cosmology

The Forgotten Realms is a sphere comprised of multitude of dimension, intricate layers that bleed into each other at points called Planes. To help mortals comprehend the scope of their reality, the planes are organized by purpose and relative effect on the Material Plane.

This interpretation is referred to as "The Great Wheel." The planes with the most direct effects on the Material Plane are depicted as closer, with the greater powers and more abstract concepts placed in rings, further from the center. The planes do not literally orbit each other in such a fashion, but this is the way most people of the realms understand it.

The Material Plane

The Material Plane is the "real world" or "Toril". But layered on top of the Material Plane just out of phase are its echoes, each a strange reflections. These echoes match the greater geography and landmarks of the Material Plane but are changed and twisted.
Feywild: Home of fairies, a place of everlasting twilight, wonder, and death. The fey are capricious, and dealing with them can range from pleasant to fatal. It is also a place of unusually potent magic.
Shadowfell: The realm of the Raven Queen and her servants, a place of despair and death, Shadowfell is a land of unending gloom. Every edifice is crumbling, and the toll of decay both physical and mental is insurmountable.

The Transitive Planes

These planes are mostly featureless and primarily function to serve as the pathway between other planes.
Astral Plane: Realm of thoughts and dreams, a sea of endless silver starlight that can only be traveled to by disembodies souls. Some souls are there exploring via powerful magic such as Astral Projection, but others are souls on their way to the Outer Planes, to the promised afterlife of various deities. Time is abstract in this place, and the denizens of the Astral Plane do not age.
Ethereal Planes: The Ethereal Planes exist solely to ensure the connections between the Material and Inner planes. This eternally fog-bound realm forms locations that correspond on each Material and Inner plane, allowing travel. The Border Ethereal is easier to reach but only allows one to view other planes. The Deep Ethereal allows travel between planes through color-coded curtains, which must be found while navigating its swirling mists.

The Inner Planes

The Inner Planes are considered to be surrounding the Material Plane and its echoes, providing raw energy and matter which created all worlds. There is a plane devoted to each raw element: Air, Earth, Fire and Water. The closer each Inner Plane is relative to the Material the more hospitable it is for visitors both in terms of terrain and denizens likely to be encountered. The further one travels in one of the elemental planes, the more abstract and dangerous things become.

At the furthest reaches of the Inner Planes, reality becomes less coherent and eventually dissolves into the Elemental Chaos. This roiling storm of chaos and raw power envelopes the Inner Planes in an eternal clash. One of the most popular locations to visit is the City of Brass on the Plane of Fire. While ruled by tyrannical and manipulative Efreeti, the City of Brass is a major economic hub.

The Outer Planes

The Outer Planes provide the direction and purpose for the raw elements of the Inner Planes. Each realm embodies a particular aspect of virtues or "alignment" spectrum. Those that embody goodness comprise the Upper Planes and, conversely, the Lower Planes are dimensions of utter evil. These planes are home to the gods of every pantheon, and loyal followers of a particular deity have their souls claimed and brought to the appropriate Outer Plane to serve out their afterlife. Celestials like Angels, Unicorns and Couatls inhabit the Upper Planes, often acting as agents for the god aligned gods. Of note is Mount Celestia, a realm of absolute justice and order, often the resting place for great champions of good. Elysium offers a perpetual safe haven, unyielding compassion, and refuge for tired souls seeking an afterlife of tranquility. Conversely, Ysgard is a rugged realm where heroes can test their mettle against harsh weather and mighty creatures that call it home.
Fiends comprise the lower planes. The militaristic Devils call the Nine Hells their home, while ravenous hordes of Demons swell from the endless Abyss. Those receiving the ultimate punishment may find themselves locked in the prison plane of Carceri with no chance of escape. Orcs are granted an eternal afterlife on the bloody battlefields of Acheron. The Lower Planes are connected by the River Styx which winds its way through them all.
Between the Upper and Lower Planes are Mechanus, the plane of absolute order, and Limbo, the embodiment of chaos. While Limbo is a formless soup of fluctuating energy and matter, Mechanus is a realm of precise clockwork gears all maintained by the curious mechanical beings called Modrons.

Beyond the Planes

Scattered throughout the planes are small pocket dimensions called Demiplanes. They are beyond count and in a constant state of flux. Some powerful wizards have been known to create demiplanes of their own.
The Far Realms lie beyond the known boundaries of the multiverse. It is home to eldritch powers, so alien in thought and form they defy comprehension. Some believe the Far Realms follow its own physical and magical laws. The Far Realms cannot be reached by a portal, but the powerful leviathans that call it home often reach into the minds of mortals. Some warlocks gain their magic through a pact with such an entity.
The Outlands technically exists between the Outer Planes. It is a realm of complete neutrality, constantly maintaining its existence in a state of paradox. At the center of the Outlands is Sigil, City of Doors. Sigil is the ultimate inter-dimensional trading post, offering goods and transport not only to other planes but allows passage between the multitude of Material Planes that exists in the Multiverse.