The Cosmology of this world is one of uncertainty. Many Planes exist merely in theory, no mortal has ever traversed such places. Similar to how Einstein conjectures a white hole exists yet nobody has seen it, several planes exist merely as variables in some great wizards arcane formula. They are the label given to explain the function of a spell, the origin of some magical force, and other phenomena.
The Outer Planes:
Material Plane: This is the real world, the Prime Material plane. You are here.
Ethereal Shadow: This is the essential plane beyond the real world. Functionally it acts as the Plane of Shadow for spell purposes. It also acts as the Ethereal plane for spell purposes. This is the strange misty world of ghosts, demons and spirits. Theologians, Wizards and lay people alike have ventured a guess as to its nature, and some have even made contact with it. This world is filled with mirrored realities of the material plane, dreams, premonitions and the thoughts from the material plane can sometimes take real form here. This plane is linked to the material world and to all other planes known and unknown. It is the plane between planes. It's true nature, shape and form is difficult to say. It is said to be made up of infinite demi-planes, pocket worlds and mini-realities. However a few have been understood and described, but they are again mere theories. Some have visited this plane and returned, describing colorful realms where fey spirits hide from the material world, others have described misty wastelands with horrid monsters and the wandering dead. Some say the plane thinks, it knows you, others say it doesn't
The Cosmic Chaos: This plane acts as the "Astral Sea," for spell purposes. The Cosmic Chaos is the raw twisting plane of magic. Separated from the Astral Tapestry by its lack of intelligence, this is essentially the binding glue of reality and existence altogether. This plane exists purely in theory, justifying some spells that allow one to transport to mysterious other planes of existance. It is considered both a seperate plane, and intricatly woven into every other plane of existance. As this is the plane of raw elemental magic in its purest, most chaotic form.
The Astral Tapestry: This plane is less a plane and more a consciousness. It is a name given by some to a plausible heaven or source of Intelligence driving the universe. Some have allegedly gotten a glimpse of it but it is said to have caused blindness or confusion. Some theorize that it is not a plane but in fact an over-soul or collective consciousness in the universe. Gods and Goddesses allegedly live here, or are part of this plane. Theologians debate heavily as to the nature of this plane, or even if it exists.
The Inner Planes:
Elemental Plane(s): Said to be the plane(s) of elemental beings and forces. Some divide it up based on elements like Earth, Air, Fire, Ice, ect... some say it is one undivided plane where the elements swirl about in chaos.
Positive/Negative Energy Plane(s): While purely theoretical, these planes are considered firmly ground in arcane sciences. Believed to be linked to the Outer Astral Tapestry, and in some cases alleged to be the engine of life and creation in the universe, akin to a power source, it is not too well understood beyond its link to divine magic and the Astral Tapestry.