The Astral Plane is an open, weightless, infinitesimal plane that connects with all other planes of existence and can be used for transportation among them (and thus described as a transitive plane, like the Ethereal Plane and the Plane of Shadow). It is the plane of thought, memory, and psychic energy; it is where gods go when they die or are forgotten.
Certain spells (such as astral projection) allow access to this plane. In the standard DnD PnP realm, a PC can enter the Astral Plane and then determine his destination. In the Forgotten Realms, a PC must declare his destination before going astral, and his destination may not change until he reaches his declared destination.
The corpses of old gods remain adrift in this timeless plane, and some beings such as the githyanki build their cities atop these "god-isles."