A Goa'uld who was outcast by the System Lords, Sokar was one of the most powerful of the Goa'uld, and chose to take on the persona of the Devil. He has posed as the god of death on many different worlds. On Earth, is identity took the form of the Judeo-Christian fallen angel Satan. It is said that thousands of years ago, when the Goa'uld occupied Earth, Sokar's host body was that of an Unas.
Before his death Sokar had amassed the most powerful fleet of any Goa'uld, and was days away from launching an all-out attack on the System Lords -- one that the Tok'ra believed he would have won. He was, in fact, at war with many other Goa'uld. His plan was to attack several key System Lords in order to tip the tenuous balace of power in the galaxy in his favor.
One of his most noteworthy enemies was the Goa'uld Apophis. After the loss of a significant portion of his forces, Apophis was captured and tortured by Sokar. He escaped and fled to Earth, demanding sanctuary -- but Sokar followed his prey and nearly destroyed the iris guarding Earth's Stargate (using a directed particle weapon). Sokar threatened to destroy Earth if the humans would not return Apophis to him. After Apophis's death, they did so.
Sokar's homeworld was Delmak. He terra-formed the nearby moon of Ne'tu to fit the myth of hell, exposing its molten core to make its atmosphere hot and chemically toxic. There he sent his captured enemies, and enjoyed regularly hearing tales of their torture and suffering by his appointed warden, Bynarr.
The Tok'ra Jolinar of Malkshur once tried to overthrow one of the System Lords, and was defeated only when Apophis joined the battle against her. Based on the fact that Jolinar spent time in Sokar's prison (before SG-1, she was the only one ever to have escaped), this System Lord may have been Sokar himself.
When SG-1 and the Tok'ra Martouf went to Ne'tu to free the captured Jacob Carter, they learned of Sokar's massive army and his plans to attack the System Lords. The Tok'ra launched a plan to take out Sokar, firing a weapon into the core of Ne'tu to cause it to explode. Sokar's ship was in orbit, and he was destroyed.
Sokar possessed a personal defense shield, a technology that is fairly rare among the most powerful Goa'uld. He was very intelligent and relentlessly harsh to his enemies. Upon his death, Apophis seized control of his massive army.
Serpent's Song - When Apophis flees to Earth, Sokar attacks the S.G.C. by firing a particle accelerator at the iris. Earth hands over their dead prisoner to avoid destruction.
Deadman Switch - SG-1 is captured by a bounty hunter working for Sokar, who nearly turns them over to the Goa'uld.
Demons - The team finds a Medieval Christian village on another world, who live in fear of the Devil, Sokar, and must give him sacrifices in the form of host bodies.
Jolinar's Memories - SG-1 travels to Sokar's hellish moon of Ne'tu, where Jacob tells them that Sokar's massive fleet is poised to attack the System Lords and conquer the galaxy.
The Devil You Know - Resurrected for the sake of torture, Apophis fails to kill Sokar -- but seizes control of his army after a bold Tok'ra plan destroys Ne'tu and kills Sokar.