From Wookieepedia: The Sith Order was a sect of Force-sensitives who utilized the dark side of the Force. The term "Sith" originally referred to a species of a

From Wookieepedia: The Sith Order was a sect of Force-sensitives who utilized the dark side of the Force. The term "Sith" originally referred to a species of alien native to the planets Korriban and Ziost, who were later enslaved and ruled by exiled Dark Jedi from the Galactic Republic...Following centuries of interbreeding and mixing of cultures between the aliens and the exiles, the Sith would no longer be identified by their race, but by their dedication to the ancient Sith philosophy. Furthermore, before Darth Bane (existing something like 1000 years before the destruction of the Death Star), who instituted the Rule of Two, there were many Sith running around (somewhere around 20,000 and even after the Rule of Two, many Sith Lords took other apprentices in secret). So there weren't always only a Sith Master and a Sith Apprentice.