Faerûn is a fictional subcontinent, the primary setting of the "Dungeons & Dragons" world of "Forgotten Realms". It is described in detail in the "Forgotten Realms Campaign Setting" (2001) from Wizards of the Coast, and various locales and aspects are described in more depth in separate campaign setting books. Around a hundred novels and several computer and video games use the Faerûn setting.
The author of Dungeons and Dragons took a great deal of time creating Faerun.