In order to unlock factions in Rome: Total War you must defeat those factions in the Imperial Campaign. They will then become available for selection when you begin a new campaign.
The playable factions are:
Julii, Brutii, Scipii, Gaul, Britannia, Germania, Greek Cities, Seleucid Empire, Parthia, Ptolemaic Egypt and Carthage.
Non-playable factions are:
The Senate, Numidia, Spain, Dacia, Thrace, Macedon, Pontus, Armenia, Scythians and rebels.