How is the Roman Republic compared with the Roman Empire?

Rome was a small city state in the 6th century and was governed by kings. However, after only 7 kings had ruled, and despite the small population size of Rome, the city managed to overthrow the monarchy and create a republican government that was supposedly supposed to represent the wishes of the Roman citizens. In order to rule the city a senate was developed, and this council was put in place to rule over Rome. Once Rome became a republic it grew in strength and went on to conquer swathes of the Mediterranean.  The Roman republic was very successful, and continued for five centuries.