What is Greek ampitheatre?

Answer 1

An ampitheatre was were the Romans held gladiator games. The biggest was the Colosseum in of course Rome. It was started by emperor Vespasian in 69 ad and successor and son Titus finished it. Others were big but most held around 13,000 people.

Answer 2

It's presumably either the original or designed from the theatre in Epidaurus in Greece.

It's an open air theatre with incredible acoustics (even the back row can hear the actors without difficulty or undue strain on actors' voices). It's the ancient Greek equivalent of the Roman Colosseum in Rome - their crowning achievement in sites meant for public presentations (a lot less violent a place, too).