The Sith Lords are an ancient order of those who embrace the Dark Side of the Force. They are the chief rivals of the justice-loving Jedi Knights.
Once long ago, thousands of Sith Lords existed and sought only to destroy, intimidate, and dominate, all the things the Jedi fought against. It is said to be founded by a rogue Jedi who tried in vain to convince the Jedi Council that the Dark Side of the Force was more powerful. He was expelled from the Knights and vowed to have revenge. He gave himself to darkness and founded the Sith Lords. Their order, though, was in many ways like that of the Jedi Knights: Force strong individuals were trained in the ways of the Sith and taught of the Dark Side. Once a pupil became a Master, he could then train another. The Sith ranks were more organized however and instituted higher ranks of Lords and clan leaders.The Sith, unlike the Jedi, had not the respect for other Lords, patience, nor any interest in a common prosperity. They began to squabble as individual Lords became unloyal to the Order and desired more power. Fighting broke out and eventually, the Sith Lords virtually extinguished themselves. Only one among them survived the ordeal and became determined to reinvent the dead Order. Renaming himself Darth Bane, he made his plans and waited patiently for his opportunity. Eventually, he took an apprentice, declaring that from this moment forth, in order to prevent greed from once again destroying the Sith, only two Sith Lords would exist at a time. When an apprentice was done with his training and his Master had died, only then would he train another. One day they could seek revenge on the Jedi.
The Sith are renamed sometime during their training. Though different names are taken as "surnames" (ex. Vader, Sidious), the title "Darth" is a constant among Sith Lords.
Sith apprentices are trained in many of the same things Jedi apprentices are: moving objects with the Force and how to weild a lightsaber being two prominent examples. Mental discipline, loyalty to one's Master, physical soundness, and total opposition to the Jedi are taught to apprentices.
Darth Sidious and Darth Maul herald the first manifestations of the Sith in over a thousand years. Though it is not known who trained Darth Sidious, he has taken Darth Maul, whom he discovered at the age of 12, as his apprentice. Maul, whose real name is Khameir Sarin, is an apt pupil and completely loyal to Sidious.
The law of only two Sith in existence if followed closely in years to come. As we know, Darth Sidious, though as Emperor Palpatine, takes as his next apprentice the young Anakin Skywalker, fallen Jedi Knight. He is not believed to have had a pupil between Maul and Anakin. Once Anakin completely turns to the Dark Side, he gives up his identity as Anakin Skwalker and takes the name Darth Vader, Dark Lord of the Sith. Now, as planned, two Sith exist once again. The Emperor, however, ages greatly and it becomes clear to both him and Vader that his death is imminent. Vader must search for his own apprentice. He desires for his pupil his own son, Luke Skywalker, Jedi in training. He cannot, however, convince his strong willed son to be his apprentice. Luke seeks, instead, to draw his father out of the darkness and resurrect him to the light. Vader and the Emperor together cannot change his mind. When the Emperor tries to kill Luke because of his refusal, Vader intervenes and destroys the dark Master himself. Moments later, Vader dies. The Sith are now once again extinct.