The Infinite was a spaceship that was forged during the Dark Times. It was said to be powered by an Old One and that whoever found it would be granted their heart's desire. The Infinite eventually crashed on one of the asteroids of the Caris System. The parts were scattered throughout the universe and power contained within started to fade away. Four data chips were needed to find its secret location.
At some point during the 40th century, these four chip were found by Captain Kaliko, Ulysees Meregrass, Gurney and Baltazar. Baltazar murdered the chip carriers to get to the ship. He also used the Doctor and Martha Jones to get the location and hijacked the TARDIS to get there. The vessel did indeed have the ability to show people their heart's desire: it provided Martha with a romantic image of the Doctor, and untold riches for Baltazar, but the Doctor successfully prevented the vessel from showing his heart's desire. Ultimately, the Doctor destroyed the vessel. (DW: The Infinite Quest)