The Colosseum or Colosseo was orignially known as the Flavian Amphitheatre or Anfiteatro Falvio. Construction was started in 72AD by Emperor Vespasian in the grounds of Nero's private Domus Aurea.
The massive construction could seat more than 80,000 people. The games held to mark the inauguration of the Colosseo lasted 100 days and nights, during which time thousands of people and animals died.
Gladiators were prisoners of war, slaves sold to the gladiator schools or volunteers. Fights were not necessarily to the death as a defeated gladiator could appeal to the crowd and the presiding magistrate, who would signal whether he had fought well and deserved to be saved or if he should be killed. Successful gladiators became popular hero's and would live to enjoy a comfortable retirement.
With the fall of the Roman Empire, the Colosseo was abandoned and later became property of the Church. There are in fact a few graves located inside the Colosseo.