The Olympian Gods were a number of powerful, immortal beings. The majority of them were introduced in the episode Freaks and Greeks. They are based off the real life Greek gods of mythology.
The Olympian Gods are immortal beings based off real mythology. They are usually characterized by having power over an aspect of the world, whether physical or abstract. For example, Cupid is the God of Love. They dwell on top of Mt. Olympus.
The show's interpretation of the Greek gods has them as a lot more immature than the real mythology suggests, with their appearance in Freaks and Greeks has them acting like party obsessed college kids.
Greek Gods first appeared on the show with Cupid's first appearance in The Zappys!. He was attending the Zappys as one of the celebrities, having a completely different design. He then returns in Apartnership! under his current design, which is more like a traditional Fairy. Cupid has gone on to be a regular recurring character, appearing several times over the show's run.
The second appearance of a Greek God came in Pan, who was a member of Cosmo's band in the episode Boys in the Band. Cosmo brought the band back together so they could stall an angry stadium of concert goers who were waiting for Chip Skylark to arrive.
Later on, in Freaks & Greeks, Timmy wishes that he could be at a "real" toga party and ends up on Mt. Olympus with the real gods. However, he accidentally destroys their home, so they decide to party at his house instead.