Forgotten Realms is a popular fantasy campaign setting for the Dungeons and Dragons line of games. It was originally created independently of Dungeons and Dragons by Ed Greenwood in 1967, but was integrated into the tabletop roleplaying game by 1987. Many novels and video roleplaying games have been licensed to use the setting over the years, notably novels by R.A Salvatore and the games Pool of Radiance, Baldur's Gate, Icewind Dale, and Neverwinter Nights.
Forgotten Realms is a campaign setting of Dungeons and Dragons. There are many multi-verses (a collection of various universes) in Dungeons and Dragons setting. Each of these realms has its own rulesets, its own history and its own set of deities. Many rules and much history is shared between these realms but much of it is different. Forgotten Realms is one of the most popular settings of Dungeons and Dragons and it was originally created independently of Dungeons and Dragons by Ed Greenwood in 1967, but was integrated into the tabletop roleplaying game by 1987. Many best selling novels and popular games use this setting - a few notable mentions are novels by R. A. Salvatore and the game series Pool of Radiance, Baldur's Gate, Icewind Dale and Neverwinter Nights.
The name Forgotten Realms is derived from the premise that long ago the Realms were very closely connected to the Earth. But ever since the connection grew weaker and people on Earth have largely forgotten about the Realms.
The Planet of Forgotten Realms is very much like Earth, but is ruled by many races. Ever since spellplague, the two 'existences' of the planet have merged with each other. These two existences were Abeir and Toril. Together the planet itself is called Abeir-Toril. This planet is the center of many campaign settings based on its continents which vary vastly in terms of geography, culture and demographics while using the same ruleset of Forgotten Realm.
After Primordials created the planet, gods interfered causing a war of epic proportions to break out. Hence Ao seperated the two planets - Abeir in the image of Primordials and Toril in the image of gods(Astral deities). Thus Abeir is a variation of Toril in its primordial form. Sometime during the transition of rulesets from 3.5e to 4e, the worlds merged. This ended up replacing some parts of Toril with Abeir and some parts disappearing. This merge has brought back some new races and Primordials to Toril which only existed in ancient times. The merge has also affected the politics, demographics and geography in a significant way. The newly appeared Abeir in the world of Toril is now referred to as "Returned Abeir".
It is believed that parts which were replaced in Toril by returned Abeir have appeared in Abeir as merge should have affected Abeir the same way. Thus after the events of Spellplague, the world of Abeir-Toril has changed a lot due to a number of major events - perhaps all connected in some way to Spellplague.
The natural satellite of Forgotten Realms is very much like Moon on Earth with similar phases as Forgotten Realms are supposed to be based on Earth. However, the Moon is actually the body of an ancient deity which helped in the creation of worlds along with her sister Shar. However, over a conflict in which Selune reached the plane of fire to set the Sun alight to nurture the life on planet, both are in a state of war with each other. The Moon is called Selune and is the body of the deity.
Sometime later, a spell was fired from the earth in order to destroy a red star which struck Selune. This caused a trail of dust and asteroids along the orbit of Selune. This trail is called 'Tears of Selune'.