The Senate is made up of: magistrates, promagistrates, legati and ordinary senators.
Magistrates are elected officials, such as: Consuls, Praetors, Censors, Aediles and Quaestors.
Promagistrates are the provincial governors like Proconsuls and Propraetors.
The third category are the Legati, these are the Legatus of the Legion who was assigned to the Senate by one of the Consuls.
The ordinary Senators are the fourth and last group, those are former Magistrates or Promagistrates whose term of office has expired. There is one extraordinary Senator and that is the Princeps Senatus, he is a senator who was chosen by the Censors to be the chairman of the senate in absence of the consuls.
Magistrates
The magistrates are the elected officials, like: Consuls, Praetors, Censors, Aediles and Quaestors.
Promagistrates
The promagistrates are the assigned officials like: Proconsuls and Propraetors. These are assigned by the Consuls and are always ex-consuls or ex-praetors. Basically promagistracy is prolongment of their term of office, with the exception that their jurisdiction is a province and not the city of Rome itself.
Senators
The senators are former magistrates. The senators have a vote in the senate and therefore have legislative power, but no executive power.
Elections
Elections for Magistrate offices and priestly offices like that of the Consul, Praetor, Censor, Aedile, Quaestor and Pontifex Maximus are held every season when there is a vacant position available, or when the term of office of one these offices is about to expire. Whoever gets the most votes at the end of the season wins the election and aquires the office. Every male character who is not a slave and who is of the required minimal age is eligible for the office, and thus may be a candidate in an election.
The elections of the Consuls, Praetors and Censors are held at the Comitia Centuriata, while other elections are held at the Comitia Tributa.
Every player has 1 vote in the Comitia Tributa, while in the Comitia Centuriata every player has as many votes, as he or she has adult male family members who are not slaves.
The election of the Censors may be moved from the Comitia Centuriata to the Comitia Tributa with the enactment of the Lex Publilia.
Imperium Investure
A Consul or Praetor has to be invested with Imperium, if he wishes to command a Legion.
Senate Decrees
The Senate can also issue decrees, while these are not actual laws as they are not stated in the Senate Legislation they do somewhat act as laws, albeit informal ones. The decree acts as a form of edict or as official policy.
War Declarations
War declarations are somewhat similar to decrees, except that they do allow for actual actions, namely hostile actions against a barbarian foe targetted by the war declaration. Therefore only the Senate can actually declare war. A war declaration may remove some of the limits of magistrates such as Consuls, effectively making them dictators.
Laws
The Senate can enact and ratify laws. For more information on these posibilities see the chapter about Senate Legislation.