In the Star Wars saga, the Sith Order is the main antagonist. Begun 6,900 years before the Battle of Yavin, the Sith are the practicioners of the Dark Side of the Force. Marks of a Sith included large, full-body tattoos, and they usually carried red lightsabers. Notable Sith include Darth Vader, Darth Tyrannus (aka Palpatine), Darth Bane (creator of the Rule of Two), and Darth Krayt (creator of the Rule of One).
"Sith" wasn't coined by George Lucas. It is used by Edgar Rice Burroughs to describe a large Wasp-like creature the size of a bull in the book the "Warlord of Mars". This term may have been picked up later by Lucas who was heavily influenced by early pulp melodramas such as Burroughs' Mars series.