The Void was the empty space between universes. "The Void" was only the name given to it by the Time Lords, it was also known to the Eternals as "'the Howling", and to others as "Hell."
Rassilon was (amongst other titles) "Ravager of the Void". (BFA: Neverland)
It contained no temporal or spatial dimensions, but language which employs them may be used to conceptualise it. For example, the Doctor confirmed that the Void had no 'up' or 'down', but also talked about the universes within it being 'stacked' on top of each other. Even "time" in most senses, doesn't actually exist there (DW: Army of Ghosts) If someone traveled through the Void, they would absorb background radiation, called Void stuff. (DW: Doomsday)
Void travel
A Void Ship was a spaceship which can travel through the Void into other dimensions. While originally believed to be only theoretical by the Time Lords, it was eventually discovered that the Daleks were in possession of a powerful Void Ship in the form of a golden sphere. (DW: Army of Ghosts)
The Cult of Skaro travelled to the Void to escape from the impending destruction of the Dalek Empire in the Last Great Time War by the Doctor, taking with them the Genesis Ark, a prison ship containing millions of Daleks, and hid in the Void until the time was right. (DW: Doomsday)
This Void Ship caused a crack in the universe, making travel from one dimension to the next easier, allowing both the Doctor's TARDIS (not a Void ship) to travel to the dimension of the parallel Earth Cybermen, and those Cybermen to travel to this Earth and invade. (DW: Rise of the Cybermen, Army of Ghosts)
After the Battle of Canary Wharf raged throughout London, the Doctor sucked all of the Cybermen and Daleks into the Void, without anyone else falling in (DW: Doomsday). The actions of other Daleks would later weaken the dimensional walls, allowing some the Cybermen to escape the Void. The rest along with the Daleks perished according to the Doctor as a result of the events surronding the Reality Bomb. (DW: The Next Doctor) Though a cybus cyberman has been seen in the trailer for series five , indicating some others escaped, some have guessed however that the cyberman seen in series five escaped from the vortex the tenth doctor sent them to at the end of The Next Doctor, via the cracks in time, rather than the void.
When the Eleventh Doctor is for a time erased from existence by stepping into the cracks, River Song describes him as being condemned to the Void between worlds.(DW: The Big Bang)
Other information
The memory changer "Adam Smith" said he had been trapped in the Void for a long period of time. (TW: Adam)
- It is not confirmed if this was the same 'Void'. If not it may be the White Void.