The Void is any part of the SL world that does not have a simulator. It is what lies beyond the "edge of the world".
The Void is not traversable under normal circumstances (one can enter the void when the client is suffering from a poor or dropped connection).
It is easy to confuse the void with "Void Sims", because of the name similarity and the fact that both look like empty ocean (usually), but they are not the same thing at all. The distinction is that a void sim does have a simulator running it and can be entered like any other sim. It is deliberately part of the world.