The Planes of Existence are a collective term for the different regions of reality that make up the Universe. The structure of the Planes are impossible for a Mortal mind to comprehend, with each Plane existing separately from each other, yet intimately linked within the same Universe.
The Universe is split into 13 different Planes, that are all connected by The Astral Sea. The Planes are split into four distinct categories, the Material Plane, the Spirit Planes, the Elemental Planes and the Divine Planes.