The TARDIS, or Timey-wimey And Related Doohickeys In Space, was a type of starship that was capable of traveling through time and space. It could look like anything, from ordinary blue phone boxes to starfighters, and was often bigger on the inside than on the outside. The Mist Hunter was a TARDIS.
There was also a variation that could only go to where the pilot has already been. It was known as a Re-TARDIS.