The Sith Creed
Peace is a lie. There is only passion.
Thru passion, I gain strength.
Thru strength, I gain power..
Thru power I gain victory.
Thru victory my chains are broken.
The Sith are followers of the Force's Dark Side
Originating from the Jedi Knights own fallen members, the goal of the Sith is revenge and domination.
Darth Bane introduced the 'rule of two. One Master, one apprentice - not merely that one master would only have one apprentice at a time, but that only the two would exist hidden and building power, while the Jedi Order, numbering in thousands, blissfully went it's way.
Sith Powers
They have the same powers as jedi, but there are others which the jedi will not use.

Ancient Sith
The Sith began when Jedi who fell to the Dark Side were exiled from the Republic. They stumbled upon Korriban, a world filled with primitive people whose version of the Force was just considered magic. They set themselves up as rulers, practiced their dark arts and enslaved the native Massassi.
The Jedi were blissfully ignorant of them until two hyperspace explorers happened across them. Using them as an excuse, a contender for leadership of the Sith attacked the Republic. They were defeated.
Sith Rising and Falling
But as the Republic expanded, it happened across worlds connected to the Sith worlds. The dark knowledge was there. It reared up on Onderon at the same time a young Jedi named Exar Kun was growing dissatisfied with his training.
The Sith knowledge escaped, Exar Kun with some, and the rulers of the Empress Teta system with others. War broke out and many other young jedi fell to Exar Kun's influence. Also to fall was Uliq Qel Droma. The Sith won a major victory when Exar Kun ordered many of his new
followers to kill their former jedi masters. He won a second when he destroyed the suns of Ossus, destroying generations of jedi knowlege and archives.
But Uliq Qel Droma came to his senses after killing his brother Cay, who still served the light. His former lover, Nomi Sunrider blinded him to the Force. And Uliq gave the jedi the location of Exar Kun's hideout. Kun fell, but left his trapped spirit behind in the temples of Yavin 4.
Disaster struck again, when the Republic came under attack by the Mandalorians. The two leaders of Republic Forces, Revan and Malek, turned to the Dark Side and became Sith. They took many Republic ships and personnel with them. In the end, Revan returns to the light and helps destroy Malek.
Again, centuries later, the knowledge was discovered. The sith reared up, using their own training facilities on Korriban and attacked until the jedi were forced to send even children into battle. They called themselves: The Brotherhoodof the Sith. In the end, a massive battle destroyed all of the remaining Sith and many jedi as well.
But one survived. Darth Bane became the maker of a new Sith order. He founded the rule of two and took an apprentice, a young girl found by the jedi and lost in the battle. So the rule of two began and continued until the Republic fell to Palpatine.

Two there should be.
One to embody the power,
the other to crave it.
-Darth Bane
The Secret Rule of the Sith
The Sith gained power through deception. Darth Plaguis passed on his rule as Master to Darth Sidious. Sidious was also known as Palpatine.
This man gained power not only through the Dark Side of the Force, but also politics. He manipulated his way to a position of Senator from Naboo, then secretly, in Sith persona, used
disgruntled members of the Trade Federation and Senatorial greed to cause and invasion of Naboo. The young Queen in part, foiled his schemes, but succumbed to his convincing speech that Supreme Chancellor Valorum would be unable to help.
His loss: one apprentice, Darth Maul, to Jedi Padawan Obi-Wan Kenobi's blade.
His gain: leadership, trust and respect of the Republic, power over the Jedi Order, and a new interest: a young boy named Anakin Skywalker.
During this time, Palpatine took a new apprentice: Count Dooku, a former member of the Jedi Order. With Palpatine working from the top and Dooku making alliances among the trade federation and disgruntled planets, unhappy with the failure of the Republic Senate to act on their grievances, they manipulated the Republic into war. A Clone Army battled the Separatist droids, with jedi in command. The Jedi ranks were thinned in battle, and among separatist worlds their reputation was tarnished as peace keepers.
Fall of the Jedi
In the end, enough Jedi had fallen and the Jedi were close to ending the war.
Having tested Anakin Skywalker, now a full jedi knight, and manuevering him into killing Dooku, Palpatine set the final trap.
Sending Obi-Wan, Anakin's mentor, off after the vicious General Grievace and with the Jedi Council scattered, he lured
Anakin to the Dark Side promising him power to save his secret wife from her envisioned death. Anakin fell and lead the Clone troops on a slaughter through the Jedi temple, killing even the younglings. Meanwhile, Palpatine has programmed the Clones to obey Order 66. Everywhere, jedi generals were attacked by their own
troops and were killed. Only Yoda and Obi-Wan were known to escaped.
The New Empire
Palpatine announced the Jedi had rebelled and turned on the Republic. He reformed the Republic into a galactic Empire. Yoda challenged him but failed.
Obi-Wan, meanwhile, went after Anakin. Unable to pursuade him to return, they battled and Anakin lost, winding up scarred and trapped with prosthetic limbs in a walking life support system.
Anakin, now Darth Vader became the image of the Sith.
Fall of the Sith
The Emperor's downfall was Luke Skywalker Anakin's son, who had survived his wife's death. Raised as a moisture farmer, become a rebel officer and now trained as a Jedi, Luke avoided and escaped traps set for him until finally, he surrendered in order to confront his father directly. When he failed to kill Vader as Anakin had killed Dooku twenty years ago, yet won the fight, the Emperor tried to kill him. He failed because Anakin Skywalker rejected the Sith teachings and turned on him, not in vengence, but to save his son. The Emperor died, Anakin joining the Force not long after. The sith reign was ended. But their legacy lived on in the form of numerous followers of the Dark Side, including the Emperor's Hands - Mara Jade, Roganda Ismaren and Vader's followers: Shira Brie AKA Lumiya and Flint.
Sith's Return
Caedus
Over twenty years after the Emperor and Vader fell, Lumiya set her sights on Jacen Solo Anakin's grandson. The young jedi, a hero of the Yuuzhan Vong war, was seduced to the Dark Side by the fear of a vision where he battled to death with his uncle Luke Skywalker. Rejecting all guidance of the Jedi, he became leader of Galactic Alliance Forces in the war against Corellia. He took the secretly took the name Darth Caedus. The Jedi, reluctant to get involved in a civil war, found themselves forced to act over Jacen's worsening methods of war and in the end he was killed by his own twin Jaina
A New Breed
Uknbeknownst to the Jedi, a new breed of Sith was growing in secret. They were led by Darth Krayt,once Anakin Skywalker's former colleague Asharad Hett during the Clone Wars. Kept alive by Sith technology, he led a new group of being similar to Kaan's brotherhood of the Sith. Once again, they succeeded in turning the Galactic Alliance and the former Imperial Remnant against each other, then swept in and attacked the Jedi. The Jedi were scattered.

Darth Krayt and his followers.