Tribunes Profile | Tribunes Basic
In the early days of Rome, when all the citizens were in one or another of the three tribes, the Ramnes, the Tities, and the Luceres, the leader of the tribe was known as the tribunus 'tribune.' These acted as the leader of their people in civil and religious matters and also in war. When the number of tribes was increased to 30, the leader of each tribe was still called a tribune. Over time his function became more limited and specialized.
When the plebeians seceded in 494, the patricians granted them the right of having tribunes with greater power than the patrician tribal heads. These tribunes of the plebs (plebeian tribunes) were powerful figures in Rome's Republican government, with the right of veto and more.
Tribunes also continued to have a military function throughout the Republic and Empire. The 24 [see Polybius] military tribunes (who were to have already served in the military) were selected by the people, the consuls and proconsuls (usually ones who had served as military commanders), or the emperor.
Source: Exercitus entry in William Smith, D.C.L., LL.D.: A Dictionary of Greek and Roman Antiquities, John Murray, London, 1875.
A patrician, Claudius Pulcher had himself adopted by a plebeian branch of his family so he could run for the office of plebeian tribune under the plebeian name of Clodius.