The Genesis Ark was the name used by the Cult of Skaro to identify a prison ship created by the Time Lords during the Last Great Time War to hold millions of Daleks. ("Genesis Ark" is unlikely to be the name the Time Lords themselves used to identify it.) It was, like a TARDIS, dimensionally transcendental.
At some point during the Last Great Time War, the four Daleks belonging to the Cult of Skaro stole the Genesis Ark and, using a void ship, fled the universe into the Void. This brought them out of the Time War, either before it became time-locked, or it allowed them to circumvent the time-lock. Their purpose was to open the Genesis Ark and re-establish the Dalek empire.
The only way for the Genesis Ark to be activated was to be touched by a time traveller, transferring artron energy to the ark. Incapable of touch, no Dalek could open the ark by any means short of blowing up a star. When Mickey Smith touched it by accident in battle, the ark opened, releasing millions of Daleks who proceeded to join the Battle of Canary Wharf against humans and Cybermen in July 2007.
When the Tenth Doctor returned to the Void all elements that had become saturated in Void stuff, the Genesis Ark was observed being sucked towards the opening after the Cult of Skaro escaped by temporal shift. (TV: Doomsday)