The history of the sith( very short summary, it goes on for thousands of years... )
There were Jedi who left the Jedi order and used the Dark side of the force for their own gains. They found the world Korriban, on which there was a race of reptillian like people, who were called the Sith. The Dark Jedi inhabited this world and made the sith their slaves. So the Dark Jedi were then referred to as Sith Lords.
A war broke out between the Jedi and the sith, the sith being greedy and lusting for power, fought amongst each other as well as fighting the Jedi. At the final battle of the Jedi vs. Sith wars on Ruusan, nearly all the sith died and came close to extinction, but one Sith survived, he was called Darth Bane.
Darth Bane realised that the sith's own weakness was their thirst for power and started a new sith order with rules that decreed that there can only be two sith at a time : a master and an apprentice. Once the apprentice becomes more powerful than the master, he kills his old master and takes on a new apprentice. The cycle carried on for a thousand years, until we get to the time of Episode 1 where we meet Darth Sidious and Darth Maul. Of course you pretty much know how the story goes from there.
Any questions?
