The Roman Republic (Res Publica Romana) vested formal governmental powers in three separate assemblies, the Comitia Centuriata, the Comitia Populi Tributa, and the Concilium Plebis. Unlike modern chambers, these bodies combined legislative, judicial, and electoral functions, and possessed the ability to make ex post facto laws retroactively making a given act illegal. Note that the Roman Senate was a deliberative chamber, and did not possess legislative or judicial powers.
The Comitia Centuriata (Centuriate Committee) included both patricians and plebeians organized into five economic Classes (knights and senators being the First Class) and distributed among internal divisions called Centuries. Membership in the Centuriate Committee required certain economic status, and power was heavily vested in the first eighteen Centuries; the Centuriate Committee was dominated by the First and Second Classes. The Centuriate Committee met annually to elect the next year's consuls and praetors, and quintannually to elect the censors; it also sat to try cases of high treason (perduellio), although this latter function fell into after Lucius Appuleius Saturninus introduced a more workable format (maiestas).
A citizen's vote did not count in the Centuriate Committee. Rather, the individual's vote was counted within his Century and determined the outcome of the Century's vote. Because only the first eighteen Centuries were kept to the nominal size of 100 members, members of those Centuries exerted a disproportionate influence over the outcome of votes. The Centuriate Committee, originally a military assembly of knights, had to meet outside the pomerium of Rome on the Campus Martius, and was as a result extremely clumsy to convoke and manage. It was not normally used except to elect the next year's magistrates.
The Comitia Populi Tributa (Committee of the Tribal Peoples) included both patricians and plebeians distributed among the thirty-five tribes whereinto all Roman citizens were placed for administrative and electoral purposes. The vast majority of the urban population of Rome was distributed among the four urban tribes, which meant that their votes were individually insignificant; like the Centuriate Committee, voting was indirect, with one vote apportioned to each tribe. The voting was therefore heavily slanted in favor of the thirty-one rural tribes. The Committee of the Tribal Peoples met in the well of the Comitia in the Forum Romanum, and elected the aediles (curulis only), the quaestors, and the tribunes of the soldiers (tribuni militum). It conducted most trials until the dictator Lucius Cornelius Sulla established the standing courts (quaestiones).
The Concilium Plebis (Council of the People) was also a tribal assembly, but it excluded all patricians, who were forbidden to take part in its meetings. Only the tribunes of the people (tribuni plebis) could convoke the Council of the People, and it usually met in the well of the Comitia (patrician senators often observed from the steps of the Curia Hostilia and heckled the tribunes during meetings -- Roman politics were considerably more rambunctious than even the modern House of Commons). The Council of the People was the favored legislature of the Republic, although technically its laws were called plebiscites. It elected the aediles (plebis only) and the tribunes of the people, and conducted trials; the latter function fell into disuse after Sulla established the permanent courts.
Although the Senate passed senatus consulta ("the senate's advice") recommending laws and measures, these were comparable to modern United Nations resolutions, and did not actually carry any force of law (except in the case of the senatus consultum de republica defendenda, the so-called ultimate decree establishing a "pocket dictator" by directing the consuls to "take care that the Republic should take no harm"). This can be seen in the conduct of the Jugurthine War, when the Senate passed a senatus consultum extending Quintus Caecilius Metellus Numidicus's tenure as commander in chief, but the Council of the People passed a plebiscite appointing Gaius Marius in Metellus Numidicus's stead. Although Julius Caesar was named proconsul of Gallia Cisalpina and Illyricum by senatus consultum, he was given Gallia Transalpina by plebiscite.
During his consulate in 88 BC, Lucius Cornelius Sulla passed a series of leges Corneliae which radically altered the political structure of the Republic. His third law prohibited the Committee of the Tribal People and the Council of the People from considering any law unless it was sent to the assemblies by senatus consultum with a favorable "do pass" recommendation. His fourth law restructured the Centuriate Committee such that the First Class -- the senators and the most powerful knights -- had nearly fifty percent of the voting power. His fifth law stripped both tribal assemblies -- the Committee of the Tribal People and the Council of the People -- of their legislative functions, leaving all legislation in the hands of the restructured Centuriate Committee (the tribal assemblies were left with the election of certain magistrates and the conduct of trials -- but no trials could be held unless authorized by senatus consultum).
These reforms were overturned by the Populares led by Marius and Lucius Cornelius Cinna, restored by Sulla during his |dictatorship rei publicae constituendae, and were again overturned after his death. They represent one of the most wide-ranging and direct shifts in the constitutions of the Roman state during both the Republic and the Empire.