The TARDIS, or Time and Relative Dimensions in Space, is the Doctor's vehicle and home that can take him to any place and any time.
Having had the interior of its control room destroyed when the Doctor regenerated, it reconfigured its appearance and now features a clicking date display, a new central time rotor and typewriters and taps for controls, which suits the Eleventh Doctor's style. It's even given him a port for his Sonic Screwdriver. The Doctor treats the TARDIS like an old friend, calling it 'dear' or 'old girl'. It's bigger on the inside than the outside and its swimming pool may now be in its library. Or somewhere else: it'll turn up.