The Genesis Ark was the Dalek name for a Time Lord prison ship from the Last Great Time War. Slightly larger and similar in shape to a Dalek casing, the Ark was dimensionally transcendental and contained millions of Daleks.
The Cult of Skaro took the Genesis Ark and escaped the war in a Void Ship, intending to use it to re-establish the Dalek Empire after they returned to the universe in 2007. However, though the Daleks could bring the Ark to its final stage of awakening, it could only be opened by the touch of a time traveller which was a safeguard to stop the Daleks from liberating their fellows from the prison ship.
The Daleks would have blown up the Sun to force the Ark open, but first they attempted to coerce Rose Tyler to open it. During a firefight with the Cybermen, Mickey Smith accidentally laid his hand on the Ark, priming it. As the Ark required 30 square miles for activation, the Daleks escorted it out of Torchwood Tower and into the sky. High above London, the Ark opened and the Daleks within began pouring out.