Elemental Chaos

The elemental chaos is the debris of creation. One way of thinking of it is the pallet used when painting the rest of Realmspace. The various parts of the chaos are the dabs of pure colour that was never blended with others to form the nuance of the other worlds. The plane borders the ethereal, astral, and material planes, but is alien to all.

The Elemental Planes

While they're all part of the elemental chaos and it's theoretically possible to walk between them, the elemental "planes" are very distinct. Each is a singular aspect of reality, taken to an extreme. The further "into" such an area that one travels, the less there is of anything but the pure element. For example, the plane of Fire es eventually just heat that will burn any matter away.

The most referenced elements with their own planes is the classic elements: Air, Earth, Fire, and Water.