The Police Box that the Doctor travels around in is really a Type 40 time and space machine from Gallifrey, his home planet. TARDIS stands for Time And Relative Dimension in Space.

The TARDIS is dimension-ally transcendental, meaning it’s bigger on the inside than the outside. The interior exists in a different, relative dimension to the exterior. In the very first story it was established that the TARDIS usually changes its exterior appearance on each trip to blend in with its surroundings. The Doctor later stated that it was the broken chameleon circuit that caused it to be stuck in the shape of an old style police box.