What it's made of:
The Colosseum is built of travertine limestone, tuff (volcanic rock), and brick-faced concrete. The Colosseum could hold an estimated 50,000 to 80,000 spectators at various points of its history over the centuries,having an average audience of some 65,000; it was used for gladiatorial contests and public spectacles such as mock sea battles (for only a short time as the hypogeum was soon filled in with mechanisms to support the other activities), animal hunts, executions, re-enactments of famous battles, and dramas based on Roman mythology. The building ceased to be used for entertainment in the early medieval era. It was later reused for such purposes as housing, workshops, quarters for a religious order, a fortress, a quarry, and a Christian shrine.
Why it's so popular:
Although substantially ruined because of earthquakes and stone-robbers (for spolia), the Colosseum is still an iconic symbol of Imperial Rome and was listed as one of the New 7 Wonders of the World. It is one of Rome's most popular tourist attractions and also has links to the Roman Catholic Church, as each Good Friday the Pope leads a torchlit "Way of the Cross" procession that starts in the area around the Colosseum.
The name "Colosseum":
The name colosseum for the amphitheatre is attested from the 6th century, during Late Antiquity. The name Colosseum is believed to be derived from a colossal statue of Nero on the model of the Colossus of Rhodes. The giant bronze sculpture of Nero as a solar deity was moved to its position beside the amphitheatre by the emperor Hadrian (r. 117–138).