The astral planes offers both structure and chaos. It is the place between, and yet a description for all realms that it connects.
You can pretty much create or visualize anything there, giving it substance and form, through subconscious thought or conscious.
More often than not, an individuals desires to encounter something or fear of doing so manifests and provides them with exactly what they are looking for.
This makes it somewhat difficult to differentiate from independent entities and more ephemeral thought forms and the like, for some.
That's my two cents on it anyway. The description can go on and on.