They lead the Galactic Empire, the antagonists of the Star Wars series who are known for brutality, being a dictatorship, and having a strong control over peoples lives. The Sith lost power for hundreds of years, before slowly taking over the Republic, or the Jedi Government, and turning it into the Galactic Empire.
In the Expanded Universe, The Sith at the start were a different species, looking like Humans, but with red skin and small bones coming out from their heads.
The Sith Species almost all died after the Jedi committed genocide on them thousands of years before the movies. This is why some people think the Sith Empire are the "good guys", who bring law and order to the Galaxy and are just responding to the Jedi actions.