Grab whatever copy of Julius Caesar you happen to have handy. (The best editions are the Arden Shakespeare, like this one, as they contain historical and literary appendices.)
The play opens on a street in Rome, with a couple of Tribunes, Flavius and Marullus. The institution of the Tribune was created to protect the plebeians from the patricians. The Tribunes had a veto over the laws of the Senate and they were protected so that the Senators could not accost them. In Shakespeare’s Coriolanus we see them formed.