Two brothers sons of Zeus with the same mother, Leta, but different fathers. Leta was king Tyndareus of Sparta's wife, with whom Zeushad fallen in love and seduced her in the shape of a swan. By him she laid two eggs, one by Zeus containing Polydeuces(lat. Pollux) and Helen, one by Tyndareus, containing Castor and Clytemnestra. When Helen was kidnapped by Theseus, it was the Dioscoures that saved her.
Polydeuces was immortal but Castor was not. When Castor died Polydeuces begged his father to either kill him as well, or let them both be immortal. Zeus then let them spend every second day on Mt. Olympus as immortals, and every second day in Hades as mortals.
The Dioscoures were widely worshipped in ancient Greece, and it was believed they protected young men and were patrons of athletics. One sacrificed to them by putting out food. They were believed to be rescuers for those in need, and that they would turn up in times of danger as youths on white horses.
They also protected sailors, and the electrical light that can be seen in masts during thunderstorms, St. Elmo's fire, was believed to be the sign that they were with them. The star constellation Gemini was their sign, showing the way to sailors lost at sea.
In art they were depicted with the kind of conical hats sailors wore, and often stars were put next to them.