The Void is a strange, artificial universe created by aliens (the Firstlifers) billions of years ago, located in the center of the galaxy and shrouded by an event horizon more deadly than any natural black hole. In order to function, it is gradually consuming the mass of the galaxy. Watched over by its ancient enemies, the Raiel, the Void's expansion is barely contained. The two planets inside the Void known to humans are called Querencia and Bienvenido, and are inhabited by humans who arrived there thousands of years ago as counted by Void-time.
The primary feature of the Void is that it inhibits and eventually prohibits the use of advanced technology, particularly electronics (or even electricity), thus rendering the society on the planet to be similar to 18th or 19th century Europe, i.e. the most advanced technology are semi-automatic firearms.
On the other hand, the Void responds to the thoughts of its inhabitants, giving them pyschic powers, including perception at a distance (farsight, ex-sight), communication at a distance (longtalk) and telekinesis (the third hand, teekay). Inhabitants have these abilities at varying strengths, i.e. some people are stronger than other with each ability.
The Void is also inhabited by an alien species called the Skylords, who fly between the stars and are the size of mountains. They can speak to humans, and their avowed purpose is to guide humans who have reached "fulfillment" to The Heart, where the will find eternal bliss.
Near the end of The Temporal Void it is revealed that the Void has another startling ability in addition to the gift of psychic powers to its inhabitants: it stores memories of events, essentially the entire configuration of the physical contents of the void at each moment in time (i.e. the state of the Void) and can, guided by the wishes of a strong psychic, reset itself to a past state, giving the appearance of travelling back in time. In this way, dwellers within the void are able (though only one knows of this ability, Edeard) to live a "perfect" life, by going back to when things went wrong and talking action to prevent the course of events that one does not like. But doing this requires enormous energy, which is why the void devours stars and planets: to provide the energy for this pathway to bliss.