The void is a dimension directly beneath the Nether and the Overworld's surface. It is populated by a race called the Mycons, mushroom-like villagers who excel in crystal magic and crafting of all sorts.
The only way to enter is through a portal in a ravine in West Ardenvell. Also, there may be gateways between the Nether and the Void.
According to Eeebs, there are crystalline plants, "Pools of blue ooze", and "Things that whisper in your ear" as described in Nether Kitten. Not much is known as it has yet to make a proper introduction.