The Doctor's TARDIS is an obsolete TARDIS a Type 40 Mark I that he borrowed "unofficially" when he departed his home planet of Gallifrey. All the other Type 40s had long since been officially decommissioned and replaced by new, improved models, although the Master does possess a Mark II Type 40 TARDIS, and the Monk's TARDIS also appears to be a Type 40.
Exterior
The Doctor's TARDIS's chameleon circuit is jammed so it appears from outside as a 1950s style blue British police box (a phone booth designed for police communications). All TARDIS's the Doctor's included are dimensionally transcendental, thus the exterior shape matters not to the interior.
Although it is supposed to blend inconspicuously into whatever time or environment it turns up in, the Doctor's TARDIS invariably shows up in the police box shape. This is due to a malfunction in the ship's chameleon circuit, the mechanism which is responsible for changing the outside appearance of the ship in order to fit in with its environment. Despite his considerable ingenuity in other fields and his ownership of a sonic screwdriver, the Doctor has been unable to fix this problem completely; the occasional temporary success has always been followed by a return to the status quo.
Interior
Console / Control Rooms
The changing appearance of the primary console room over the years implies that the Doctor does upgrade the TARDIS's systems every now and then. The TARDIS may also be capable of periodically changing its own interior appearance (DW: The Invisible Enemy). The TARDIS also has a secondary control room, and possibly more control rooms within its vast interior.
The Doctor's TARDIS has at least two console rooms, including a white-walled, futuristic one, and a secondary one which has wood paneling and a more antique, Edwardian feel to it. Due to the architectural programs of the TARDIS, the Doctor was able to create different styles of rooms with ease (as stated in the Doctor Who 2006 annual). The main feature of the rooms, in any of the known configurations, is the TARDIS console that holds the instruments that control the ship's functions. The appearance of the primary TARDIS consoles may vary widely but share common details; hexagonal pedestals with controls around the periphery and a moveable column in the center that bobs rhythmically up and down when the TARDIS is in flight. The secondary console was smaller, with the controls hidden behind wooden panels, and had no central column. The entrance to the TARDIS can be locked and unlocked from the outside with a key, which the Doctor keeps on his person.
The TARDIS interior walls generally consists of roundel's; a circular indentation that line all of the TARDIS walls. Some roundels conceal TARDIS circuitry and devices (DW: The Wheel in Space, Logopolis, Castrovalva, Arc of Infinity, Terminus). The design of the roundels may vary depending on where in the TARDIS they are; and include a basic circular cut-out with black background, roundels resembling washing-up bowls stuck to the wall, recessed wood paneling with a few decorative ones in what appeared to be stained glass, translucent illuminated discs, and hexagonal shapes with nodes in the center.
Specific Control Systems
Specific Controls Its controls are said to be isomorphic, that is, only the Doctor can operate them (DW: Pyramids of Mars). However, various companions have been able to operate the TARDIS and even fly it. The Time Lords are also able to pilot the TARDIS by remote control, usually, as the Doctor once bitterly noted, so he may take care of "some dirty work they don't want to get their lily-white hands on" (DW: Colony in Space, The Brain of Morbius).
Specialised Control The Second Doctor once used a portable Stattenheim remote control given to him by the High Council to summon his TARDIS to him (DW: The Two Doctors). The Rani also uses a Stattenheim remote control to summon her TARDIS, though the Master's compliment to her on this point in indicates she developed hers before the Time Lords on Gallifrey did (DW: The Mark of the Rani). The TARDIS is also vulnerable to diversion or relocation by the Guardians, Eternals, and other immensely powerful beings.
Other Rooms
The cloister room on the TARDIS sounds the cloister bell when disaster is imminent. The interior of the TARDIS also exists in a state of "temporal grace", which is supposed to ensure that no weapons can be used inside its environs. This last function is also inconsistent in its application (DW: Earthshock).
Other Systems
Emergency Systems
The Doctor's TARDIS contains various emergency systems, one such is the jade pagoda, a 'life boat' of some description, which can in theory be piloted (NA:Iceberg), but in emergencies it will lock onto the nearest (spacially and temporally) planet with a breathable atmosphere and bareable climate. (NA: Sanctuary)
Defensive Systems
Some of the TARDIS's other functions include the Hostile Action Displacement System (HADS), which can teleport the ship a short distance away if it is attacked (DW: The Krotons).
Problems
Because the Doctor's TARDIS is old, it is inclined to break down. The Doctor is often seen with his head stuck in a panel carrying out maintenance of some kind or another. Thus making it reliable only when it is most needed.
Personality
At times the TARDIS also appears to have a mind of its own. It is heavily implied that the TARDIS is intelligent to a degree, and has a bond with those who travel in it. The Eighth Doctor once called the TARDIS "sentimental" (DW: Doctor Who: The TV Movie). The Doctor once even met a TARDIS which "died" after its Time Lord master had passed away (Omega).