The Sith are practitioners of the Dark Side of the Force as opposed to those who used it for good, the Jedi. The Sith are not one race in the same way as the Klingon Empire are. In the early days of the Force, the two sides of the Force fought for control of the Force with the Jedi being victorious. The Sith died out but one Dark Side apprentice used intelligence and cunning to work his way up from Senator on Naboo to become known as Emporer Palpatine.
It had been forseen that Anakin Skywalker would ultimately destroy the Sith but he chose to join them and become Darth Vader. The Sith master in the star Wars films is Darth Siduous whose apprentice is Darth Vader. Darth's master is Darth Plagueis, a powerful Sith Lord who had the power to bring people back from the dead before being killed by his own apprentice, Darth Sidious.
In Star Wars I - The Phantom Menace, Darth Sidious has an a Zabrak apprentice called Darth Maul to go out and prevent the Jedi warriors Qui-Gon Jinn and Obi-wan Kenobi to ensure that Padme Amidala didn't reach the Jedi Council on Coruscant to plead for assistance in resolving the trade dispute between Naboo and the Trade Federation. The Trade Federation was being manipulated behind the scenes by Darth Sidious.
The Sith use a system of Master and Apprentice, there are only ever two Sith Lords at any one time. The force of two was started by Darth Bane many years before any of the films are set. Bane was the sole survivor of a war between the Sith and the Jedi and he recognised that infighting between the various Siths had ultimately led to its downfall, he decided to therefore insist there only be two Siths and that became known as the Rule of Two. For someone to become the Sith Master, the apprentice would have to kill the Master.
Palpatine wanted Luke Skywalker to turn to the dark side by killing Palpatine but Luke tried to resist the urge to kill either Palpatine or Darth Vader. When the pressure got to much for Luke Skywalker, Luke took up his lightsabre and tried to strike one of them down but Darth intervened and father and son fought a battle. Palpatine wanted Luke to kill Darth so that Luke would become the apprentice but Luke managed to resist which angered Palpatine who wanted to have Luke as the apprentice.
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