TARDIS: A Brief Over-view
A TARDIS is the pinnacle of Time Travel technology of the inhabitants of the Planet Gallifrey and is quite possibly the most advanced ship in all of science fiction.
Note: Most examples in the following will be based on the antique Type 40 TARDIS used by 'The Doctor' since that is, quite simply, the TARDIS we know the most about. Information suggests that the Time Lords are currently constructing TARDIS' in the Type 90's but no reliable information exists about them at this time.
TARDIS is the abbreviation for Time And Relative Dimension In Space which in itself is a rough description of what a TARDIS really is (explained later). On Gallifrey itself, they seem to be sometimes referred to as TT Capsules but the name 'TARDIS' seems to become more and more prevalent even there. The term TARDIS would seem to have been originally coined by the grand-daughter (Susan) of a renegade Time Lord called 'The Doctor'.
Put as simply as possible, a TARDIS is a time machine; a vehicle which allows the users to go anywhere in all of Space/Time at will.
The exact method on how a TARDIS travels is unknown but it is clear that the exterior shell enters the Time Vortex and travels along it until the appropriate location is reached.
Shape & Form
The outward appearance on a TARDIS is dictated by the Chameleon Circuit, a system designed to make a landing TARDIS as inconspicuous as possible. For example, a TARDIS arriving in Ancient Rome might take the form of a stone pillar or maybe a large water feature. Likewise, a TARDIS materializing in a forest might take the shape of a tree or a boulder.
More on the chameleon circuit below.
The shape of a TARDIS not using this ability would appear to be a plain box some six feet high with one side swinging open as the door.
The door of a TARDIS is actually a dimensional bridge which links the outside shell dimensions (which exists in the dimension that humans walk around in) with the pocket dimension that the interior of a TARDIS exists in. It's this separation of dimensions which is one of a TARDIS' main defensive assets (more below).
The interior of a TARDIS in no way reflects its outward appearance. To start with, the interior is many times bigger than the exterior - estimates of exactly how large vary from the size of a large city to literally infinite space. Contents of each TARDIS seem to vary from model to model but The Doctor's was seen to contain the following; two Control Rooms, numerous bedrooms, cricket pavilion, large wardrobes, gardens, swimming pool, food machine, zero room, and a lot of corridors.
The interior seems to be able to change form just as much as the exterior; in the book 'Times Crucible' the interior of The Doctor's TARDIS was changed in an extraordinary fashion - instead of the traditional bright corridors, a landscape crossing three different time-zones (past, present and future existing next to each other) complete with skyscraper like buildings and streets. While this massive change did place considerable strain on the already damaged TARDIS in question, it does show just what a TARDIS is capable of in regards to changing its interior dimensions.
Telepathic Circuits & Sentience
A TARDIS appears to be semi-sentient, being able to communicate with its pilot on a subconscious level through the telepathic circuits as well as possessing a strong self preservation instinct. To use 'Times Crucible' as an example once again, The Doctor's TARDIS did not want to allow The Doctor entry so it simply shifted the doors on the outer shell. Likewise, the TARDIS has been known to completely remove switches/levers from the Control Console completely when it did not wish its pilot to perform certain acts. It has even gone so far as electrifying the Control Console.
A TARDIS is equipped with an impressive sensor package, able to accurately measure all manner of variables ranging from radioactivity, gravity, heat, humidity, temporal anomalies ... even radar. The sensors would seem to tie in with the TARDIS landing routines - locating a safe out-of-the-way spot for the TARDIS to land (so it doesn't squash some poor unfortunate being for example).
There is also an external visual scanner which can pan around the TARDIS and relay the images to the view screen in the Control Room.
In 'Logopolis' The Doctor is quoted as saying that 'only the exterior of the TARDIS that exists as a real Space/Time event' and the outter plasmic shell is driven by the Chameleon Circuit, so if the Chameleon Circuit is functioning properly then the outter TARDIS shell can be changed into anything.
The visible area of a TARDIS is set, as mentioned above, by the Chameleon Circuit, a device which makes a TARDIS blend in with any surrounding it should land in. Various shapes that TARDIS' have been seen to take include; a Police Box, Corinthian Pillar, Grandfather Clock, Computer server, Tomb, Cadillac, university building and even a moving statue.
This shape is automatically selected by the Chameleon Circuit just before materialization at the destination but can easily be overridden by the pilot using the Control Console (early version of specific control system seen in 'Logopolis', screencap to right). There does not seem to be any real limits upon what shape the exterior of a TARDIS can take, with The Doctor once trying to change his TARDIS into a replica of one of the Great Pyramids (his bid failed due to his faulty Chameleon Circuit but it seemed to be well within the abilities of the Type 40 TARDIS).
Defenses & Security
A fully functioning TARDIS is next to impossible to destroy; this is due mainly to the nature of its dual-dimension existence. After all, how can you possibly affect something which is located in an inaccessible other dimension? The only available target for attack is the outer shell but since that in itself is a projection of the 'real' TARDIS, it is more or less completely invulnerable in its own right. Shaking of the TARDIS interior has been seen on numerous occasions when the exterior has been attacked, this is most likely due to the dimensional bridge which links the exterior and interior together. There is only one known event which has been shown to be able to destroy a TARDIS and that was 'Event One' - and a TARDIS can even escape that by finding extra power and dematerialising away.
A TARDIS employs at least two seperate defense shield systems (while the outer shell is invunerable it's best to have a multi-layered defence for obvious reasons).
The first envelopes the entire outter dimensions and has been referred to in 'Horns of Nimon' in a manner which suggests that it is used to block such things as collisions and mundane attacks. The forcefield was also seen in 'Shada' when the Doctor used the College Room TARDIS' forcefield to journey to his own TARDIS while both were travelling in the Time Vortex. The second system is the forcefield for the doors (which have been quoted as running on a seperate system). This shield can be extended ('Horns of Nimon', screencap to right) to be used as a sort of corridor to walk from the TARDIS doors to whatever lies at the other end of the corridor.
The door to a TARDIS can only be opened from the outside by a genetically encoded key which is adapted to recognize one being. This key is also able to be used by any person the original owner grants permission to (permission is usually given by merely handing the key to someone). The Time Lords on Gallifrey possess master keys which let them into any kind of TARDIS (as seen in 'The Deadly Assassin') which only makes sense since they are the ones that built the TARDIS' in the first place.
The interior of a TARDIS has its own defensive systems; one which The Doctor once referred to as a 'State of Temporal Grace'; this particular system would seem to render any sort of attack unavailable - from energy blasters to mental attacks (as seen in 'Hand of Fear').
A TARDIS would also appear to emit some kind of temporal field in both its interior and exterior - as seen in 'The Daleks Master Plan' this field (among other things) restores negative temporal effects (the effects of the Dalek time manipulation weapon which had aged The Doctor and his companions was nullified upon reaching their TARDIS). Note; This field did not appear to be working during 'Mawdryn Undead' but due to the unpredictable nature of The Doctor's TARDIS, this is not surprising.
The Zero Room was seen in 'Castravolva' and was promoted as being a zone in which there were no outside forces at work. The Zero Room was used to help The Doctor through his difficult regeneration and it would appear to be used mainly for relaxation and healing purposes.