Nature of the Planes

The Material Plane that holds the world of Toril is one of many planes of existence.
The Ethereal Plane can be thought to wrap around the Material Plane along with its echoes, the Feywild and the Shadowfell.
Beyond the Ethereal Plane lies the Elemental Chaos, a riotous plane where elements and energy mix and clash with little rhyme or reason. As one moves deeper into the Chaos, the elements and the energy planes become more separated, eventually forming the planes of Air, Earth Fire and Water, along with the positive and negative energy planes. Planar theorists like to think of these "pure" planes as surrounding the Chaos.
Opposite the Elemental Chaos is the realm of thought and idea. The Astral Sea connects the Material Plane to the Outer Planes, which are the homes of the gods and outsiders such as demons, devils, angels, and other powers. Those who study the Outer Planes argue over whether the Outer Planes are arranged as a Great Wheel, whether they are free-floating Dominions in the Astral Sea, or whether they are connected as a Great Tree, connected by branches. Certainly planar travelers have reported climbing the Great Tree, sailing the Astral Sea, or propelling themselves through an Astral Plane by means of their own thought. Others have traveled the planes using an "Infinite Staircase" and other models. It may be that in a realms of thought and idea, one's own perceptions and beliefs determine the planar model for an individual or small group.
It is known that the Dominions of the Gods, such as the House of the Triad, are found on the Outer Planes. The Lawful Good Realm of Mount Celestia, also known as the Seven Heavens, contains the House of the Triad along with Dwarfhome.
A fourth great realm exists, sealed away for all time. The home of the Great Old Ones and the Outer Gods, the Far Realm is a place of Madness and Entropy. The creatures there seek the destruction of all reality, in order to remake it in their own image. Small ruptures sometimes occur, usually with the assistance of insane mortals. The illithids, beholders, and aboleth all trace their origins to this Far Realm.