The Sith are an order of Force practitioners who have chosen the dark side of the Force as the source of their power. Members of the Sith Order are called Sith Lords or Dark Lords of the Sith. The Sith are the sworn enemy of the Jedi, whom they have fought for thousands of years.
Sith only operate in pairs — one master and one apprentice. Though it was not always so, under the current doctrine of the Order there can only be two Sith Lords at any given time. This doctrine was put in place by a legendary Sith Lord named Darth Bane. Bane did this to prevent the destructive infighting that plagued the Sith Order for millennia. With this system, Dark Lords pass the Sith legacy through the generations in an unusual and violent way.
After years of working together, an apprentice will kill his master when he senses an opportunity to become the master himself. This new master then takes an apprentice of his own. This method ensures that only the strongest Sith survive.
The Sith Code
Peace is a lie
There is only passion
Through passion I gain strength
Through strength I gain power
Through power I gain victory
Through victory my chains are broken
The Force shall set me free
History of the Sith 