The TARDIS is the main means of travel for The Doctor. It is also the main type of ship used by the Time Lords, the alien race of the Doctor, and can travel instantaneously to any point in time and space. This is achieved through The Time Lords' ability to harness the power of black holes in their technology. It's most notable aspect is that the interior is vastly larger than the exterior, and it is often said that a TARDIS can house an infinite amount of 3-dimensional space. This allows a TARDIS to house an artificial black hole at it's heart to become it's primary source of energy. Another notable aspect is it's ability to blend into any environment through the use of a chameleon circuit, which automatically transforms the exterior of the ship into a disguise according to the present era and planet.
The Doctor's TARDIS
The Doctor's TARDIS is stuck as the shape of a 1950s-style London police box when the Doctor visited earth in the 1960's. This was apparent when he visited a pre-historic earth directly after being in London, which lead to the discovery of the broken chameleon circuit.
The Doctor's TARDIS is also a decommissioned Type 40, stolen from his home planet, Gallifrey. Among the Time Lords, it is considered obsolete.
Even though there is more than one TARDIS in existence, during the overall length of the series it is usually is referred to as The TARDIS. Also The Doctor has referred to it as "the finest ship in the universe." Ever since the destruction of his planet and race, this has become a painful truth for him.
The Console Room
The TARDIS Console room is the most well-known area within the TARDIS, as the ship is controlled from here. Over the years, the Console room has changed in layout and look, often - but not always - with a change in Doctor. The TARDIS Console is six sided (allegedly due to the fact that it requires six pilots to steer it correctly) and often has many weird and wonderful devices on it, such as bicycle pumps, telephones, typewriters and many switches, knobs and levers.
The rest of the Console Room varies in design according to the current incarnation of the Doctor. In times past, the walls were white and lined with roundels (although a rarely used "backup" Console Room was wood-lined), but in more recent series these have been replaced with more organic pillars and other structures.
The TARDIS in Gaming
As well as appearing in Doctor Who games, The TARDIS also made a cameo appearance in Fallout. When the player character approached the TARDIS in the 'Dr. Who' Special Encounter, the blue police box would dematerialise, accompanied by the familiar grinding noise of the ship's engines, leaving a motion detector for the player to pick up.