The Void is the 4th dimension. It is populated by a race called the Mycons, mushroom-like villagers who excel in crystal magic and crafting of all sorts. The void is also populated by crystalline plants and "things that whisper in your ear," along with "pools of blue ooze." Since Mycons have the Shadowsight racial bonus, it can be assumed that the Void is darker than the 1st dimension.
Reaching the Void
From the Overworld, one way to enter the Void is through a portal in a ravine in West Ardenvell.
From the Nether, one can scale down huge holes. They are not physically connected, rather at the bottom of the hole is a massive portal.