[QUOTE=Lumpy]
Dark Jedi != Sith Lord. I gather that “Sith” means a member of a particular exclusive school or heritage of the Dark Side.
[/QUOTE]
First of all, I can’t believe I know this. I’m such a geek. But this all got explained in the Star Wars “extended universe”…in other words, all the books, comics, etc. that Lucas licensed to milk the great cash cow that is Star Wars. So, here’s the history of the Sith
Anyway, it seems a long, long, long time ago (even longer than the long, long time ago referenced in the movies), there was an argument among the Jedi, with some of them saying that the whole “Dark Side of the Force” really wasn’t that bad after all, and this argument turned into a civil war among the Jedi. The Dark Jedi lost, and the rest of the Jedi said, basically, “get lost”, and exiled them from known space.
The dark Jedi travelled through the unknown reaches of the galaxy, until they came upon a planet called Korriban. Korriban was inhabited by an intelligent species, known as the Sith, who could use the force, but didn’t really understand it. When the dark Jedi came, the Sith worshiped them as gods, and the dark Jedi (being dark Jedi, after all) enslaved the Sith. This went on for a while, with the dark Jedi interbreeding with the native Sith, and meanwhile expanding their control into nearby solar systems, so that “Sith” stopped being the name of the species, and started being the name of the growing Empire. The Sith Empire was ruled by the most powerful of the Dark Side force users, who held the title “Dark Lord of the Sith”, and usually got to his position by killing his predecessor. As you can imagine, this isn’t something that leads to stable succession.
One of the most powerful Dark Lords was this guy named Marka Ragnos, and in fact, he was so powerful, he managed to die in bed, which was something that most of his predecessors weren’t able to do. But that meant, when he did die, there were two strong contenders for the throne. Meanwhile, a Republic ship went off course, and crashed on Korriban, carrying with it two Jedi. One of the rivals for power manipulated the situation, and managed to become Dark Lord by convincing the Sith to declare war on the Jedi and the Republic. The Republic won and the Sith empire was destroyed. As time goes on, there are other uprisings of dark Jedi, but they keep getting put down
So, jump ahead to a few hundred years before the movies. This disaffected Jedi falls to the dark side and takes the name Darth Bane. He decides the reason the dark side keeps losing is because it’s members keep betraying each other (which is, in fact, a large part of the reason. Being both powerhungry and egotistical, the dark Jedi have trouble putting aside personal gain for the grater good). So, Darth Bane decides that from now on, the Sith should be limited to one master and one apprentice. When the apprentice has become stronger than the master, he will kill him, and take a new apprentice of his own. This way, the Sith can survive, in secret, waiting for the day the Jedi and the Republic become weak.