Revan's Sith Empire granted this title to its high ranking members, with the Dark Lord of the Sith as the most powerful and their leader, however, the majority of the dark sided users were known as Dark Jedi.
Those who aspired to become a Sith Lord had to train at the Korriban's Sith Academy and pass several brutal trials that would test their prowess and strength in the dark side, if they succeeded, they were admitted in the Order as a new Sith Lord.
The New Sith Empire under Darth Ruin was in power during the Dark Ages of the Jedi. Sith Lords then stalked the galaxy, until the organization eventually fell to infighting, and broke into individual Sith Lords.
Lord Kaan's The Brotherhood of Darkness gave all Sith Lords nominal equal status among the Sith. This strategy was created to prevent the Sith from killing each other, which had been the natural order of the dark side up until the formation of the Brotherhood.
At the end of the first war between the Jedi and Sith, Bane saw this as a violation of the natural order of the Sith and cleansed the Order by destroying Lord Kaan's Brotherhood and supplanting it with his own Rule of Two; two Sith, one master to embody the power and one to crave it. In most cases, the apprentice was trained until he or she was strong enough in the Force to kill his or her master.
After the Galactic Civil War, Darth Krayt abandoned the Rule of Two to rebuild the Sith, thus multiplying the population of the Sith Lords hugely, all under the rule of Darth Krayt. They then attacked the Empire after the Sith–Imperial War and ruled the galaxy.
Originally, "Sith Lord" referred to a high-ranking member of the Sith race's society. As the Sith civilization progressed, many claimed the title and battled against each other to earn the superior title of Sith'ari: the Sith Overlord; Adas and Dathka Graush were such Overlords.