it's divided in three periods: MONARCHY, REPUBLIC & EMPIRE.
MONARCHY = the power is in the hands of a king. it started from: 753 b.c. to 509 b.c.
REPUBLIC = the power is in the hands of a group of representative. From 509 b.C. to 31 b.c.
EMPIRE = the power is in the hands of an emperor. From 31 b.c. to 476 a.c.