In 509 B.C. the Romans formed an entirely new form of political organization called the Roman Republic. The three main types of bodies that were the components of the constitution of the Republic were the Magistrates (officers of state), the Senate, and the popular assemblies: the Assembly of the Soldiers, the Assembly of Tribes and the Plebeian Council.
The Magistrates were officers of state and served as the executive branch. The were five types: the consuls, the praetors and the censors were senior officers and the aediles and the quaestors were junior officers. The consuls were the heads of the city and the army. The praetors could also command an army. There was not a centralised form of government and the magistrates operated independently within the remit of their office. The were elected annually except for the censors, whose term of office was originally five years and later 18 months.
The Senate was an advisory body, but could also issue decrees. It was not an elected body. Originally its seats were taken by members of the patrician aristocracy and former consuls. Later the other former senior offers of state also automatically became senators With Sulla's reform in 82 BC, the senate became composed exclusively of former magistrates, both senior and junior.
Originally bills were proposed by the consuls and voted on by the Assembly of the Soldiers which represented the citizen-soldiers or the Assembly of the Tribes which was open to all citizens. Later the representatives of the plebeians, the plebeian tribunes, became the main proposers of bills and the plebeians council (the assembly of the plebeians) was the body which voted on most of the bills.