During the Titanomachy, the war between the Titans and the Olympian gods, Prometheus sided with Zeus, helping to overthrow the old gods. Siding with the winning side, Prometheus avoided being punished with the rest of the Titans and was therefore not sent to Tartarus, the Underworld.
In all accounts, Prometheus was presented to be the protector and benefactor of mankind. He offered a number of gifts to humans, including fire. Moreover, he tricked Zeus by asking him to choose between two offerings; beef hidden inside an ox's stomach (something pleasing hidden inside a repelling exterior) or bones wrapped in glistening fat (something inedible hidden inside a pleasing exterior). Zeus chose the latter and hence, a precedent was created in what humans could sacrifice from that moment; so, they kept the meat for themselves and sacrificed bones to the gods.
Zeus was infuriated and punished Prometheus by having him chained to a rock, where an eagle ate his liver during the day, and the liver was regenerated during the night due to Prometheus' immortality. He was later saved by the demigod Hercules.