The letters TARDIS are an acronym, standing for Time And Relative Dimension In Space. A TARDIS is a Time Lord's time and space machine and represents the summit of Gallifreyan technology. It is, in effect, an artificial universe of potentially infinite size, with computing power so great that it can decipher the whole of the past, present and future of our own universe. Not surprisingly each TARDIS is an intelligence, albeit an artificial and alien one. It has the power to materialize a part of its physical structure anywhere in space and time, and it can adapt its appearance to blend in with the surroundings in which it materializes. The TARDIS is said to be dimesionally transendental which is just a technical way of saying that it is are bigger on the inside than it is it is on the outside.
The Doctor's TARDIS is a Type 40, an obsolete model which has a number of interesting features that are missing on more recent versions. The Doctor's TARDIS is also in need of an overhaul: hardly any of its circuits work properly and the chameleon circuit, which allows the TARDIS to change its appearance, has broken completely: the Doctor's TARDIS is stuck in the shape of a blue police telephone box of the sort that used to be a common sight on London's street corners a few decades ago.