The Ethereal Plane is one of the Inner Planes in the Great Wheel cosmology. In fact, it is the second-innermost of the Inner Planes, being one of the Planes closest to the Prime Material. Described as a grey and misty realm that exists just slightly out of phase with the physical universe, the Ethereal Plane is mostly ignored, save for the fact it's the "default plane" for ghosts, and that a skilled wizard or other caster can traditionally use the Ethereal Plane as a way to get around physical obstacles.
It's generally perceived as a silvery, weightless place, with minimal physical matter. The physical plane can be dimly perceived from the ethereal plane, but ethereal things generally can't interact with physical things or vice versa unless they get, for lack of a better word, "close," into the parts of the ethereal plane right next to the physical. This is called the "border ethereal."
Out in the "deep ethereal," time slows down (along with the metabolic processes of anyone travelling there, fortunately), and proto-matter bubbles around. This is where wizards go for raw materials when they're building their own demiplanes.
|The Cosmology of Planescape|
|Inner Planes||Ethereal Plane||Prime Material||Astral Plane||Outer Planes|
|Elemental Planes||Energy Planes||Demiplane of Dread||Plane of Shadow||Plane of Mirrors|
|World Serpent Inn||Tu'narath||Sigil||Demiplanes||Ordial Plane?|