A Void Ship was a vessel designed to exist outside Time and Space, thus being capable of traversing the Void that existed "between" all parallel universes and dimensions. A Void Ship could protect its occupants from "the Big Bang, the end of the universe and the start of the next", and, until it properly landed in its target dimension, could defy all forms of analysis, meaning it had no detectable mass, heat, quantifiable age or radiation.
Due to these properties, people who looked upon a Void Ship found it deeply unsettling; foreboding. An invisible barrier prevented anything from the outside from having physical contact with the ship. (TV: Army of Ghosts)
In the Doctor's universe, Void Ships were considered by the Time Lords, including the Tenth Doctor, to be impossible to build and strictly theoretical, (TV: Doomsday) although TARDISes, and, through them, the Time Lords at the height of their power, were able to travel between parallel universes through different mechanisms. (TV: Inferno, Rise of the Cybermen, etc.)
The Doctor, at any rate, held that the technology held by the Cybermen of Pete's World in the early 21st century was insufficient for it to even be plausible that they would build a Void Ship; he described the technology involved as "way beyond [them]". (TV: Doomsday) When these Cybermen eventually did gain the ability to traverse the dimensional barrier, it was done using a Dimension Vault they'd stolen from the Daleks. (TV: The Next Doctor)
The first true Void Ship in the Doctor's universe was the Sphere, constructed by the Cult of Skaro, who had foreseen the destruction of Dalek-kind at the end of the Last Great Time War and opted to run away so as to avert their species' extinction. (TV: Army of Ghosts)