According to the setting's creators, Forgotten Realms is the name of an imaginary fantasy world
that exists somewhere beyond the real world. The setting is described
as a world of strange lands, dangerous creatures, and mighty deities,
where magic and seemingly supernatural phenomena are quite real. The
premise is that, long ago, the Earth and the world of the Forgotten
Realms were more closely connected. As time passed, the inhabitants of
planet Earth have mostly forgotten about the existence of that other
world—hence the term Forgotten Realms.
The focus of the Forgotten Realms setting is the continent of Faerûn, part of the fictional world of Abeir-Toril, usually called simply Toril, an Earth-like
planet with many real-world influences. Unlike Earth, the lands of the
Forgotten Realms are not all ruled by the human race: the planet Abeir-Toril is shared by humans, dwarves, elves, goblins, orcs,
and other peoples and creatures. Technologically, the world of the
Forgotten Realms is not nearly as advanced as that of Earth; in this
respect, it resembles the pre-industrial Earth of the 13th or 14th
century. However, the presence of magic provides an additional element
of power to the societies. There are several nation states and many
independent cities, with loose alliances being formed for defense or
conquest. Trade is performed by ship or horse-drawn vehicle, and manufacturing is based upon cottage industry.
Source: Wikipedia