The TARDIS (Time And Relative Dimensions In Space) is The Doctor's 'vehicle' used for going through space and time, however it is not just a 'vechicle,' it has been implied many a time that the TARDIS contains some life force or sentience. It is bigger on the inside than on the out, which is time-lord technology, playing about with dimensions. The doctor does not own the TARDIS strictly speaking, and he often refers to the fact the he borrowed it from Galifray, when he left, and always intended to return it, however since Galifray is destroyed and the Time-Lords (nearly) all dead, this seems unlikely. It looks like a blue police telephone box, at the moment, however it used to be able to blend into its surroundings, having a plasmic outer shell, but the mechanism for the has become stuck. The Tardis has several control rooms, and the Doctor changes around the ones he uses. The TARDIS's controls and the Doctor's driving tecnique are wild, eccentric and complicated, often the Doctor seems to have little control over it, and they end up in a different place entirely from where they were intending to go.
The TARDIS is extremly secure, and although it looks like a Police phone box, many of the Doctor's foes have been unable to gain acess, due to a powerful force field, that can deflect even a Dalek's balster. During the Doctor's travels, he and his companions, (but since we don't know the extent of the Doctor's education, we can't tell if this is for his benefit too) are privy to the language of all the aliens they meet, which is just as well, or it wouldn't make very good TV. As was mentioned earlier, the TARDIS is extremly large inside, (mentioned to be similar in size the the Empire state building.)
The TARDIS is mentioned to contain (and we see some of these too.) A swimming pool, the console room (obviously), a kitchen, several passages connecting rooms, a large closet, living quarters, an attic, a library, several bathrooms, a scullery, squash courts, laundromat, zero room (for the Doctor's eaceful regeneration.)