I wondered this before myself, I found that [Wikipedia had most of my answers][1]. 

> The Forgotten Realms is a fantasy world setting, described as a world
> of strange lands, dangerous creatures, and mighty deities, where magic
> and 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 name
> Forgotten Realms.

  [1]: https://en.wikipedia.org/wiki/Forgotten_Realms