The spirit realms are generally descrided as dimensions or planes distinguished by the different densities and vibration rate the substances they contain, and the various dimensions are said to interpenetrate, existing in the same space. Different people have divided these infinite realms into different hinders of vibrating regions and various names have been used by different people to differentiate amount these regions. For most of Us living in the physical world and preoccupied by its material comforts, those are concepts so it may be easier to visualize the spiritual planes when you concentrate.
The physical plane is composed comparatively dense, slowly vibrating matter.
The astral planes are generally divided into the lower, middle, and higher, although each actually contains many, many divisions of varying of vibration rates. After we shed our physical body, most of us reportedly "wake up" in our astral body living in the specific astral plane in which substance is vibrating at the same rate as the substance of our astral body.