Senatus Populusque Romanus (Senate and People of Rome) are a non-playable Roman faction.
This faction is unique, as it only occupies one region (Rome) and does not try to conquer others. It sends the other Roman families on missions to increase the power of the Roman empire, and is the eventual target for all factions.
In multiplayer the SPQR have identical units as the House of Julii Romans. In the Imperial campaign the SPQR will give Roman players missions to complete such as taking settlements and killing enemies of the Republic. These missions offer rewards such as units, games held in your honor, and money.
The SPQR seems not to be able to produce any post-Gauis Marius event units except for the First Legionary Cohort's.