Forgotten Realms
The "Campaign Set" (often referred to as the "Grey Box") was later released in 1987 as a boxed set of two source books ("Cyclopedia of the Realms" and "DM's Sourcebook of the Realms") and four large maps, designed by Greenwood in collaboration with author Jeff Grubb. The set introduced the campaign setting and explained how to use it, and reserved space on the map for SSI's Gold Box computer role-playing games set in the Forgotten Realms.
Fantasy world
"Dungeons & Dragons", the first major role-playing game, has created several detailed and commercially successful fantasy worlds (called "campaign settings"), with established characters, locations, histories, and sociologies. The Forgotten Realms is perhaps the most extensively developed of these worlds. These elements of detail can be a large part of what attracts people to RPGs.