The geometry shown here is representational rather than absolute. The Ethereal permeates all the infinite number of Prime Material planes, including Prime 0 (the plane containing the universe the world of Alair exists within), and links them to the six Inner Planes (the four Elemental and two Energy) which actually surround the Ethereal equally and totally.
Connecting the Ethereal plane to the Outer Planes, and also permeating the Inner Planes, is the Astral Plane. Arranged around that are the Outer Planes, otherwise known as the God Planes; these are the homes of the various Gods, devils and demons - each pantheon has their own realm of one or more infinte planes.
Getting Around
There are several means of travelling from one plane to another. Of course, getting there is one thing; surviving is another. Arriving on the Elemental Plane of Fire without protection, for example is pretty suicidal. It can also be a bad idea to arrive unannounced on a God Plane without being sure one is welcome.
* Plane Shift. This spell will relocate up to eight people from the plane they start on to any other. Simple as that.
* Ethereal Jaunt,Etherealness. These shift the character (and a few others in the latter case) onto the Ethereal plane. From there, they may either return to one of the Prime Material planes or exit onto one of the Inner planes.
* Astral Travel.This projects an image - the Astral Body - onto the Astral Plane, from where it can emerge onto any of the Inner or Outer planes. The Astral Body is joined to the character's real body - which remains where it started - by the Silver Cord, a thin tough strand which fades to invisible about 5' behind the Astral Body. If the Astral Body is killed, the character simply "wakes up" back in their own body. If the real body is killed, or the (extremely tough) Silver Cord is cut, the character is killed.
* Gate. The Gate spell briefly opens a two-way portal to the desired plane, and allows the caster the option of asking a denizen of that plane to step through (there is no compulsion to do so, but the caster can attempt to control the called creature if it does).