Apollo was the son of Jupiter and Leto, and twin brother of Diana. He had a number of different functions and symbols, and could be presented in two very different ways: on the one hand, he incarnated rational order and a type of masculine beauty related to that rationality. On the other hand, he was also a vengeful god who could be cruel and murderous. He could punish, and inflict illness on those who did him harm (for example, he killed the children of Niobe, who had insulted his mother Leto). Yet again, he could also heal and purify (he was the father of Asclepius, god of medicine).
He was also the god of music and poetry, especially of musical harmony, and was often accompanied by the Muses (the nine goddesses of the Arts and Sciences). He was sometimes associated with the Sun (as his sister Diana was with the Moon), and was then called Phoebus ("Phoibos" in Greek).
In Antiquity, Apollo was above all famous for the temples that were dedicated to him in Greece. The best known is the temple at Delphi, where Apollo was worshipped because he had rid the region of a fearsome snake, called Python. This temple then became known for its oracle, who communicated the god’s will, and for the Pythian games, instituted to commemorate Apollo’s heroic feat.
Apollo was very popular with the Greeks, and his popularity was revived in the Roman world when Augustus, the first emperor, chose him as his personal patron god in the late Ist centry BC.