Well, to make a nerdy TARDIS-based lecture...
Originally, the TARDIS was utterly buggered, and sent the doc and his companions on compeletely unpredicatable paths through time and space. Then, following the third Doctor's release from his exile on Earth, it worked ok for a bit, but gradually got more and more buggered 'cos of a lack of maintainence, since it had only a crew of one (or, tops, two) Time Lord when it needed a good deal more. Remotely, though, the Time Lords on Gallifrey could operate it all they liked, presumably via the space internet or something.
Famously, of course, the chamelion circuit has always been buggered, leaving it looking like a police phone box. Except that one time where COlin Baker managed to turn it into a Hammond Organ for a while. That crazy guy.