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. Of course, should the apprentice fail to kill his master, he will surely be killed in retaliation and the master will find a new apprentice. This method uses the Sith Lords' innate ambition to make sure that only the strongest Sith survive.
Unlike the Jedi, who use their own names, Sith Lords take a title that starts with Darth (the ancient Aurabesh pronunciation for the word for "dark") and that ends with a name of their choice, such as Darth Vader or Darth Stuffo. This was not always the case with the Sith. It's a system used by modern Sith Lords to hide their true identities.
Like the Jedi, the Sith are skilled in the art of lightsaber combat. Some of the most legendary Force swordsman have been Sith Lords. In recent history, Darth Maul was considered one of the most talented and savage warriors ever to wield a lightsaber. Maul used a custom-built, double-bladed lightsaber and could easily fight several Jedi at once.
In the Jedi Order, each Jedi student makes a journey to get Illium Crystals for the lightsaber they will one day build. This journey is considered a rite of passage. As a result, Jedi sabers are many different colors depending on the crystals they find.
There is no such ritual for the Sith. The Sith prefer synthetic crystals for their sabers. The synthetic crystals save the Sith the long trek to get Illium Crystals, provide more blade tuning options, and give Sith sabers their characteristic red blade.
After thousands of years and many wars, the modern Sith Lords have learned to use deception and subterfuge rather than outright warfare to further their quest. This shadowy, manipulative approach is what ultimately led the Sith Order to destroy the Jedi Order and take over the galaxy during the rise of the Galactic Empire. But as with all things, none of this would have been possible without the boundless power afforded to them by the Force.