A trial was a formal examination of the actions of an individual, seeking to determine whether the individual had infringed some form of law. It could be held in a court, with the term of court applying both to the physical location of the trial, to the trial itself, and to the judging panel.
Officials
On Earth, trials were typically presided over by a judge. (AUDIO: Judoon in Chains) However, Gallifreyan trials could also be headed by a small group of representatives of the High Council, (TV: The War Games) by the Lord President (PROSE: The Legacy of Gallifrey) or by an Inquisitor. (TV: The Mysterious Planet)
Lawyers, or attorneys, would represent the accused, seeking to defend them via rhetorical argument directed to the judging court. (COMIC: The Court of Birds) In turn, a prosecutor would present an argument for the accused's guilt. On Gallifrey, "valeyard", a name adopted by a maleficent emanation of the Doctor, was an obscure term for a particularly learned prosecutor. (TV: The Mysterious Planet)