The Astral Plane is the featureless aether in which all other planes float. It is often used for travel between and within planes, and creatures which are ethereal in another plane are partially manifested in the astral plane.
No creatures dwell natively in the astral plane, though githyanki and mind flayers, among others, have been known to sail it in special ships. There is no air to breathe on the Astral Plane, you must bring your own.
Rumour has it that if one sails in one direction on the Astral Plane long enough, one might come across the enormous, non-Euclidean Crystal Sphere that encircles all the planes.