An ancient order of Force-practitioners devoted to the dark side and determined to destroy the Jedi, the Sith were a menace long thought extinct. Thousands of years ago, a rogue Jedi had come to the understanding that the true power of the Force lay not through contemplation and passivity. Only by tapping its dark side could its true potential be gained. The Jedi Council at the time balked at this new direction. The dark Jedi was outcast, but he eventually gained followers to his new order.
With the promise of new powers attainable by tapping into the hateful energies of the dark side, it was only a matter of time before the order self-destructed. Power-hungry Sith practitioners fought amongst themselves and dwindled their numbers. Weakened by infighting, the Sith were easily wiped out by the Jedi.
One Sith had the cunning to survive. Darth Bane restructured the cult, so that there could only be two -- no more, no less -- a master, and an apprentice. Bane adopted cunning, subterfuge, and stealth as the fundamental tenets of the Sith Order. Bane took an apprentice. When that apprentice succeeded him, that new Sith Lord would take an apprentice.
Thus, the Sith quietly continued for centuries, until the time of Darth Sidious...