Real world interface

The real world interface, on some occasions referred to as the door, was the part of the TARDIS which connected the outer plasmic shell of the ship to the interior.

Functions
By crossing the dimensional threshold of the interface a person or object underwent an instantaneous translation from the ongoing space-time continuum occurring outside the vessel to that pertaining in the separate micro-environment within (or vice versa). This process was known as "going into the TARDIS" or "coming out of the TARDIS". (TV: Logopolis)

Depending on how the chameleon circuit configured the TARDIS, the real world interface might actually resemble a door or doorway. On other occasions, the real world interface might not be visible at all, making the TARDIS appear to have no obvious entrance or exit.

Opening the doors while the Doctor's Type 40 was in transit usually had grave consequences (TV: Planet of Giants, Warriors' Gate), but once materialised, the TARDIS force field usually protected the interior even if the doors were opened to a hostile environment. For example, if the doors were opened in a vacuum, the interior of the ship would not undergo explosive decompression. (TV: Four to Doomsday, The Runaway Bride, The Stolen Earth)

Real world interface of the Doctor's TARDIS
The Doctor's TARDIS originally had what appeared to be two sets of doors, the police box doors of the outer shell, which were manually operated from within and without, and a pair of more substantial doors covered on both sides in the usual roundel motif. The inner doors were usually power-operated — operating a switch on the console or opening the outer doors manually caused the inner doors to open automatically. A dark void linked the two sets of doors. The pressure exerted by the door servos was not especially great; a Cyberman was able to force the doors open against the mechanism. In the event of a total power failure, the doors could be cranked open and shut manually. (TV: Death to the Daleks, Earthshock, PROSE: Full Circle)

By the time he met Rose Tyler in his ninth incarnation, the Doctor's TARDIS had a simpler arrangement, with the police box doors of the ship opening directly onto the console room. The doors were usually manually operated during that era. Despite the appearance of simplicity, the Doctor believed the protection provided by the doors when closed to be exceptionally strong, telling Rose that "the combined hordes of Genghis Khan couldn't force their way in here — and believe me, they've tried." (TV: Rose) On one occasion, when the TARDIS' exterior shell shrank, so did the door on the interior. (TV: Flatline)

River Song revealed that the future Doctor she knew habitually opened the TARDIS doors by clicking his fingers, something the Tenth Doctor said was impossible. However, shortly after her death, he tried this and found it worked. (TV: Forest of the Dead) He did not routinely use this ability in the rest of this incarnation, although his eleventh incarnation did later demonstrate it to Amy Pond. (TV: The Eleventh Hour)

The real world interface could vanish entirely with no way into or out of the TARDIS once the TARDIS entered siege mode. This could not be circumvented manually, and required additional power to "turn it off". (TV: Flatline)