As the Romans never had a written constitution it is difficult to neatly fit their government into anything that corresponds to modern systems.
However, especially from the time of the passage of the Lex Hortensia (287 BC), it is similar to that of the American division of executive, legislative, and judicial branches, as the Roman system was the basis for the American one.
The system developed into one of checks and balances with the aristocratic Patrician dominated Senate being counterbalanced by popular elections of chief magistrates and the Plebeian Tribunes.
Did you know...?
Lex Hortensia was the formal end to Plebeians vs. Patricians.