If you per chance happen upon a large blue box about the size of a phone booth that very much resembles a 1950s earth style Police Box in an unusual location, it may not, as seems probable, be something that belongs in a museum (or a television studio), but rather a spacecraft. Called a Tardis, for Time And Relative Dimensions In Space, this often shape shifting type of spacecraft is a product of the Time Lord race, which originated from the planet Gallifrey. If you are not in 1950s England on the Planet Earth when you find this particular Tardis, then it most likely belongs to the Doctor, the last currently known surviving member of the Time Lord race. The Doctor’s old and outdated Type 40 Tardis is most often in the form of a 1950s Police Box (Which was used by the police around English cities as a direct line to a police station and as a temporary holding cell) for the simple reason that, at one time when the Tardis traveled to 1950s England its chameleon circuit (which changes the Tardis’s external appearance to inconspicuously blend into its surroundings) malfunctioned and failed to change the external appearance of the Doctor’s Tardis after leaving 1950s England. Since then the Doctor has fixed his Tardis’s chameleon circuit, but has apparently grown fond of the Police Box exterior, and has mostly kept it in that form.

The fact that you’ve found this Tardis is particularly astonishing however because finding a Tardis is tricky. The Tardis employs a perception filter to reinforce beliefs that it is a perfectly ordinary object. Obviously you have deemed this not an ordinary everyday run-of-the-mill object. Though just to be certain that it is indeed a Tardis, and not a museum piece, you should wait around to see if it dematerialises. The dematerialization will be accompanied by a sound described by some as a cyclic wheezing, groaning noise and possibly a wind as the Tardis fades slowly from view on its way to another time and space. If however, you have just seen it materialize you may be in luck, because the Doctor will most likely step out of it, and be able to help you with any alien infestation problems you may currently be experiencing on your planet.

Sources: http://en.wikipedia.org/wiki/TARDIS