The modern Olympics present themselves as a revival, and the most useful thing to know about the ancient festival is how little it resembled a sports meeting.
It was a religious festival for Zeus. The competitions were held within it, not the other way round. A sacred truce was proclaimed so that competitors and spectators could travel through warring territories, and the central event of the five days was not a race but the sacrifice of a hundred oxen on the altar of Zeus, followed by a feast. Winners dedicated their victories, and their prize was a wreath of wild olive cut from a sacred tree, the material rewards came later and at home.
The altar itself makes the point better than any text. It was not built. It was a mound of ash from previous sacrifices, mixed with river water and packed, growing year on year for centuries until it stood several metres high. The sanctuary's holiest object was the accumulated residue of everything that had been offered on it.
The statue. The temple of Zeus housed Pheidias's seated figure of the god in gold and ivory, some twelve metres high, counted among the wonders of the world. Ancient visitors describe an effect closer to awe than to admiration. It was taken to Constantinople and destroyed there. Pheidias's workshop at Olympia was identified in excavation and confirmed by a cup found in it inscribed with his name, about as direct a link between a named ancient artist and a surviving building as archaeology offers.
The end came with Theodosius I's suppression of public pagan worship at the close of the fourth century AD, after a run of something over a thousand years.
One modern practice should be labelled as modern. The lighting of a flame at Olympia by parabolic mirror and its relay to the host city is not an ancient rite and has no ancient precedent. It was devised for the Berlin games of 1936, by the organisers of those games, and has been performed ever since as though it were inherited. German excavation at the site, incidentally, began in 1875 under Ernst Curtius on terms that left the finds in Greece, unusual for the period, and the reason the temple sculpture is at Olympia rather than in Berlin.