The Roman Empire was founded as a city-state in the 8th century BC. According to Roman myths, the city of Rome was founded on April 21, 753 BCE. In the beginning Rome was a kingdom.
The Roman Republic was founded in 509 BCE. From this time until Emperor Augustus, Rome was ruled by a senate of privileged families, who each year elected 2 Consuls to lead the republic.
When Augustus, born Octavian, was given the power to rule he established a new form of goverment. This was called the Principality - where he himself was Princeps, meaning first-citizen in the Roman Empire (Hence the title "Prince"). He was also elected Dictator for life and gave himself the nickname of Caesar, after his uncle Julius Caesar. In Latin, Caesar is pronounced Kaesar. Controlling Rome's armed forces gave him the title Imperator, or "Emperor" and Augustus had thus created what is subsequently called the Empire.
The classical Roman Empire stood united until the year 293 AD where Emperor Diocletian divided the realm into 4 parts with 3 co-emperors. This was again changed with the death of Emperor Constantine the First in the year 332 AD. Thereafter the empire was divided into 2 parts; a Western and an Eastern Roman Empire.
The Western Roman Empire fell in the year 476 AD, when Goths and other tribes from the north conquered the Empire and took Rome. The Eastern Roman Empire - also called the Byzantine Empire - lasted until the year 1453 when Muslim armies conquered the capital Constantinopel and founded the Ottoman Empire.