There were 10 tribunes at any given moment, and they might act individually or collectively. These tribunes were given the authority to utilize the People's Assembly (the Concilium Plebis) to help the common people. They could summon the Senate, propose laws, and intervene in judicial cases on behalf of the plebeians. The most famous example of a tribune who used this authority to good effect is Marcus Tulius Cicero. Other examples include Spurius Cassius, Lucius Domitius Ahenobarbus, and Gaius Marcius Coriolanus.
The power of the tribunes was not absolute. They could not pass laws, only recommend them. Also, they could not create courts or offices, only propose bills for these purposes. Finally, they could not declare war or make treaties; these were powers reserved to the Senate alone. However, by giving the tribunes the power to utilize the People's Assembly, the Senate gave up some of its own power as well.
In addition to their power to propose legislation, which was limited to subjects within the province's competence, the tribunes could also veto measures proposed by the Senate and elected officials within their jurisdiction. If two-thirds of the senators voted against bringing back a vetoed measure, then it would be considered defeated and would not go into effect.
A common misconception is that the power of the tribune was equal to that of the senator.
These tribunes had the authority to convene and preside over the Concilium Plebis (people's assembly), summon the Senate, propose legislation, and intervene on behalf of plebeians in legal matters; however, the most important power was the ability to veto the actions of the consuls and other magistrates, thereby protecting the plebeians. The tribune Tiberius Gracchus introduced many reforms that improved the status of the common people, including land redistribution. When his nephew was murdered by a senator, Tiberius then proposed several more amendments to the Constitution, some of which were accepted and some of which were not. He was later killed by a senator who feared that Tiberius would continue his campaign for reform.
The power of the tribunes gradually decreased after this event, until they were finally abolished in 287 BC.
These tribunes had the authority to convene and preside over the Concilium Plebis, summon the Senate, propose legislation, and intervene on behalf of plebeians in legal matters; however, the most important power was the ability to veto the actions of the consuls and other magistrates, thus protecting the plebeians' interests. The right of appeal to the people from their judges was also available to them.
Tribunes could also declare war or make peace. They could also command armies in wartime. However, they could not give military orders to anyone other than a consul or another tribune. The power of declaring war or making peace was an important one for which several examples can be given. For example, after the Battle of Lake Trasimene in 217 BC, two of the three Roman generals surrendered because they could see that there was no hope of victory anymore. Yet, their opponents would not accept this surrender and so the generals were killed. This shows that even though the tribunes had the power to order armies into battle they could not win the wars by themselves. Only politicians could do that. However, they could protect the plebs from being harmed by such decisions.
In conclusion, tribunes held great power within the Roman society because they could defend the interests of the plebs. If anything harmful was about to happen to the plebs then a tribune would step in and prevent it.