Tribune was the title given to three separate offices during the Roman Republic: the Tribune with Consular Power, an office of the fifth and fourth centuries B.C., obsolete by the time of the Third Servile War; the military tribune, officers ranking between the centurions and the legate commanding each legion; and the Tribune of the Plebians, the most powerful office of the Roman Republic, although most of its powers were negative, blocking affimative actions by the magistrates or the Senate.
During the early Republican era, in times of war, up to four legions would be levied and trained. Twenty-four military Tribunes, six for each legion, would be elected to act as the legions senior officers and assist one of the Consuls, whom would act as the overall commander of two of those four legions.
After the Marian Reforms, and with the increase in the number of permanent professional Roman legions which were raised by the Senate (and in some cases private citizens), the original overall number of twenty-four military tribunes naturally increased for each new legion, so distinctions were made for tribunes from the Senatorial and Equestrian orders of Roman society. The Tribunus Laticlavius (broad-stripped tribune) was a son of the Senatorial class, whom from the age 18-20, were elected to their position, and as part of learning curve for men of their status, would act as the official second in command for the Legatus Legionis. For every Laticlavii in a legion, there were five tribunes Angusticlavii. An Angusticlavius was a member of the Equestrian order who would be appointed to his role after serving some time as an Auxiliary Prefect, or as a soldier in the Eques Legionis (120-strong legionary cavaly cohort). The Angusticlavii were often staff officers with no fixed command, though they could sometimes be entrusted to command a Vexillation, which was a temporary mixed force of both legionary and Auxiliary cohorts. Angusticlavii, given their prior experience were often somewhat older than their Laticlavii superiors, though, Angusticlavii would see their position as a step toward a career in the Roman Senate.
The Tribunes of the Plebians were the champions of the plebian class, elected by the Committee of the Plebians, identical in organization to the Committee of the Tribes, but consisting only of the plebians of each tribe. The Committee of the Plebians elected ten Tribunes of the Plebians to intercede against the acts of the Senate and the magistrates. The person of a Tribune of the Plebians was sancrosanct, protected from any assault or indignity, and could extend this inviolability in the form of a veto against any act of the Senate or magistrates. This power to paralyze the government amounted to a greater power than wielded positively by the Senate and magistrates, and the Tribunes of the Plebians were regarded as the most powerful political figures of the Roman Republic.