The Phandelver Irregulars
While precise details of the universe is unknown, the Celestial Spheres can be traveled by individuals of great renown.
The Inner Sphere consists of the Prime Material Plane with all the worlds in the known universe. Surrounding the Prime Material Plane is the Etherial Plane, a misty realm of proto-matter.
Outside of the Inner Sphere is the Elemental Planes, one for each of the four prime materials, earth, fire, water and air. The Elemental Planes float in a pool called the Astral Plane.
The Outer Planes, also called the Planes of Power, are 16 planes arranged in a circle (the Great Wheel) defined by alignment and surrounding a 17th neutral plane known as the Concordant Opposition.