Ra, the deceased sun god of Abydos, represented Earth's first exposure to the Goa'uld. Originally one of the most powerful and influential of the Goa'uld System Lords, Ra was responsible for finding humans on Earth (the "Tau'ri") and discovering that the Goa'uld could use them as hosts, maintaining their bodies indefinitely with their advanced technology. Ra stumbled upon the planet Earth thousands of years ago, and took an aboriginal boy as his host.
Ra is believed to be one of the aliens responsible for taking humans from Earth and seeding them all across the galaxy. These human colonies worship the Goa'uld as gods, serve as slave labor (such as on Abydos, where the people mined naquadah for Ra) and have no idea that their true origins lie on Earth.
As a powerful System Lord, Ra's enemies were many. He was mortal enemies with Apophis, his brother in ancient Egyptian mythology. He was also opposed by the Goa'uld Shaq'ran, and defeated the Tok'ra queen Egeria, who spawned a resistance movement within the Goa'uld. He came into conflict with Earth when the first Stargate team visited Abydos. After attempting to destroy Earth by sending a naquadah-enhanced nuclear bomb through the Stargate, Ra was killed by Jack O'Neill and Daniel Jackson when they transported the bomb to Ra's mothership -- which was fleeing Abydos after a rebellion by its people.
Ra had a bride, Hathor, whom he exiled to Earth. She spent thousands of years trapped in a sarcophagus before being discovered by archaeologists. With Hathor, Ra was the father of Heru'ur, a powerful Goa'uld System Lord in his own right.
"Stargate" the Movie - Ra is killed when Jack O'Neill and Daniel Jackson lead the Abydonians in rebelling against his oppressive rule, thwarting his plan to destroy Earth.
Children of the Gods - Earth fears that Ra may have returned when a Goa'uld attacks the base and kidnaps an officer, but learns that it was Apophis.
Hathor - O'Neill and Jackson win praise from Hathor when she learns that they killed her pharaoh, Ra.
Cure - SG-1 visits Pangar, previously controlled by Ra, and find that the Goa'uld imprisoned the Tok'ra queen Egeria there, rather than killing her.
Moebius, Part 1 - SG-1 travels 5,000 years into the past to steal a Z.P.M. from Ra, who ruled on Earth at the time.
Stargate: Continuum - After Baal changes the past, even Ra himself lives and is subject to him.