The Far Realm is one of the more obscure planes of Dungeons & Dragons, and basically amounts to the obligatory "Cthulhu Mythos Dimension" for Lovecraftian horrors. It is formally considered an invention of 3rd edition, although its roots lie in obscure D&D lore from the very beginning, in particular the adventure "The Gates of Firestorm Peak", which actually named the plane. 3rd edition merely expanded on the details most highly, which, combined with the fact it isn't formally placed on the Great Wheel, is why most consider it a 3e invention.
The Far Realm is generally described as, or believed to be, the origin-point for all aberrations (although a moment's thought would indicate that aberrations could only be touched by the Far Realm at some point in their evolutionary history, as native entities actually plane-shifted out of the far realm drive PCs insane just from being looked at). The exception is in Eberron where they are instead the creation of the daelkyr.
Ironically, the Far Realm was about the only major plane to survive completely unchanged when 4th edition switched from the Great Wheel to the World Axis. It is still the mysterious alien realm from which aberrations are believed to spring and about which little is known. It's actually quite important to the Nentir Vale setting; shardminds were born when a cosmological construct keeping the Far Realm at bay was acidentally broken by some of the gods, whilst the energies pouring through that heavenly wound are suggested as one possible source for psionics.
|The Cosmology of Planescape|
|Inner Planes||Ethereal Plane||Prime Material||Astral Plane||Outer Planes||Ordial Plane?|
| Demiplane of Dread
Plane of Shadow
| Plane of Mirrors
World Serpent Inn
|The Cosmology of the World Axis|