The prequels develop “the Sith” as a secret order of people in positions of power who manipulate events in hopes of taking over the galaxy and have the same powers as the Jedi, but use these powers for evil. They have “Darth” names, use red lightsabers, and (at least Sidious and Maul) wear black versions of Jedi robes. We’re told that they seemingly went extinct 1000 years ago. ROTS reveals that they ruled the galaxy before they went “extinct”. Yoda says that there’s always two, no more, no less, a Master and an Apprentice. Interviews with Lucas and the novelization explained that the Rule of Two was started by someone called Darth Bane in response to infighting.