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". In addition to this, it has an overgrowth of fungus and crystals that can be found throughout the mycelium-carpeted, floating islands. Since Mycons have the Shadowsight racial bonus, it can be assumed that the Void is darker than the first dimension, the Overworld.
Reaching the Void
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.