The Malfeasance Tribunal was a body within Time Lord society that had the power to pass sentence in accordance with the galactic laws.
Following the end of the War Lord's trial, the Doctor was charged with having repeatedly broken the Time Lords's most important law of non-interference in the affairs of other planets. Having accepted his plea that there is evil in the universe that must be fought, they also noted his particular interest in Earth and the special knowledge of its problems that he had acquired on his frequent visits. The tribunal chose, in view of the extenuating circumstances, to impose a lenient sentence: banishment to Earth, where he would continue to defend the vulnerable planet. He would be sent to Earth in the 20th century and remain there for as long as deemed proper, with the ability to operate the TARDIS being taken away from him. As part of his sentence, he was also to have his appearance changed. In an order by the tribunal that was dated 309906, one Type 40 TT capsule - the Doctor's TARDIS - was removed from the register, leaving 304 deregistered and non-operational capsules from the 305 originally registered. (DW: "The War Games", "The Deadly Assassin")
With the Doctor in the midst of his exile, the tribunal thought that he should be made aware of the danger posed by the arrival of the Master on Earth. They knew that the Master would certainly try to kill him, so a messenger was dispatched to warn the Doctor. (DW: "Terror of the Autons")
When a cosmic energy drain initiated by an unidentified hostile within a black hole endangered the time travel facility of the Time Lords and threatened the destruction of the whole fabric of space-time, the Doctor was sent, with the help of his two previous incarnations displaced from their parts of the timestream, to reverse it. With the crisis averted, the tribunal's sentence was subsequently remitted at the intercession of the Celestial Intervention Agency. (DW: "The Three Doctors", "The Deadly Assassin")