Pelias is Jason's power-hungry uncle who kicks Aeson, Jason's father, off the throne of Iolcus. Pelias is pretty much a total jerk, but if you take a look at his childhood you can get a peak into where his jerkiness might come from.
Along with his brother, Neleus, Pelias was abandoned by his mother, Tyro, on a mountain. See, Tyro had been tricked into conceiving the boys by Poseidon, who had disguised himself as her lover, the river god Enipeus. Later on, Tyro married Kretheus, King of Iolcus, with whom she had Aeson.
Raised by shepherds, Pelius grew up totally dissatisfied. Shepherding? What kind of life is that for a son of Poseidon? The bitter young man was determined to be a king. So, as soon as he could, he went and took the throne of his half-brother, Aeson.
Along the way Pelias killed Tyro's stepmother, Sidera, who at one point had been mean to Tyro. Of course, he made a really big mistake when he did this, because he killed Sidera on the altar of Hera. The queen of the gods was not what you'd call forgiving, and she made it her mission to make his life miserable. Hera's hatred of Pelias is a big part of the reason she helped out Jason.
Pelias goes down in a long list of wicked kings whose main purpose seems to be to make some young handsome hero's life miserable. He reminds us a lot of King Eurystheus, the guy who assigns Heracles his Twelve Labors. However, just like with Heracles, Jason's quest ends up making him famous. In the end, Pelias is brought down by his own devices.