The place where it all happens. The Forgotten Realms was published in 1987 as the first official setting of DnD. The main setting of the Realms is the Sword Coast, the western coast of the continent of Faerun on the planet Toril. The Sword Coast consists of several city-states with peace maintained by an alliance of lords. Magic exists, and permeates every aspect of daily life. Danger lurks in every corner, on every road. Normal folk never go off into the woods, because any number of monsters may live there that might have any number of reasons to make a villager's life unpleasant. Adventureures are a rare breed of people that take advantage of this dangerous world for glory, or gold, or whatever they wish really. The Realms are expansive, and as a player in Faerun, there is detail all around you.
More information about the forgotten Realms can be found at the Forgotten Realms wiki.