The Planes

The Planes represent all of reality in Dungeons & Dragons. There are many theories as to the logical arrangement of the planes. Most learned persons subscribe to the Great Wheel cosmology, discussed here.

Introduction
A "plane" is an extremely large area that has certain physical characteristics. There are broadly several types planes.
 * Prime Material Plane: This is the normal world. Planets exist here, such as Toril, on which the continent of Faerûn sits.  The worlds of the Prime are planets only in the sense that they are spheres of terrestrial size.  They do not orbit suns; stars and other astronomical phenomena are the work of gods.
 * Inner Planes: Surrounding the Prime are the elemental planes. These four greatest of these are the classical elements: Fire, water, earth, and air.
 * Outer Planes: While the Inner Planes and Prime are composed of elements which form their environments, the outer planes are formed of ideas given form. This is where angels, devils, demons, and gods reside.  Each Alignment has its own plane.  Many outer planes have "layers," which while sharing the same alignment as the parent plane, manifest it in different ways; e.g., there are both scorching and freezing parts of Hell.
 * Astral Plane: Think of this as outer space, and the other planes planets and stars. The astral plane is what everything else exists in, and is mostly empty space.

Outer Planes
Each alignment has its own matching plane, with several that are "inbetween" alignments. Good Neutral Evil
 * Lawful Good: Seven Mounting Heavens of Celestia. Often simply called a "Heaven," this is a vast city, ruled by order and goodness.  It is the most heavily fortified place in the planes, standing as a bulwark against the endless armies of the lower planes.
 * Neutral Good: Elysium
 * Chaotic Good: Arborea
 * Lawful Good/Lawful Neutral: Arcadia
 * Lawful Neutral: Mechanus. A vast clockwork of gears, some continent sized, moving according to perfect order.  Some sages believe Mechanus is actually a giant machine that powers the planes.
 * Neutral: The Outlands. A mostly empty space between all the other Outer Planes.  At its center, on top of a great spire is Sigil, the City of Doors.
 * Lawful Evil/Lawful Neutral: Acheron
 * Chaotic Good/Chaotic Neutral: Ysgard
 * Chaotic Neutral: Limbo
 * Chaotic Evil/Chaotic Neutral: Pandemonium
 * Lawful Evil: The Nine Hells of Baator. Here those who were wicked in life are given their just punishment.  It is ruled by devils, who fight the endless Blood War with demons.
 * Neutral: Tarterian Depths of Carceri
 * Evil: Infinite Layers of the Abyss. The Abyss is much larger than any of the other Outer Planes, being composed of hundreds (some say 666) layers, each ruled by its own demon lord.  The Abyss fights the Blood War against Hell.