Lord Zack's Planescape
The Astral Plane is the primary spiritual plane of existence. It connects the Material Plane with the higher planes of existence, such as the realms of the gods. Close to the Material Plane you can see the Material and interact with it to a degree. It is in this area that living spirits and also undead spirits like ghosts and other incorporeal undead exist.
There are some spirit realms that exist close to the Material Plane, often close to portals to higher spiritual realms or places with high spiritual significance.