The Sith must be ruled by a single leader, the very embodiment of the strength and power of the dark side. If the leader grows weak another must rise to seize the mantle.
—Darth Revan's holocron
A Dark Lord of the Sith was the highest ranking Sith title achievable. The Dark Lord of the Sith was known as the head of the Sith, whether it be under a Sith Order, the Rule of One or the Rule of Two. A Dark Lord of the Sith was the most powerful of all Sith, hence his position in an order. All Sith under him would be considered Sith Lords, regardless whether it was one apprentice or an entire group of them. This continues for many decades until the rise of Darth Bane who believed in the rule of two. Before him there were many Sith lords who wield the force. They were powerful in their own right, though they killed themselves to often for any of them to be supreme. This was one of the factors that lead to the continuous fall of the Sith.
It should be noted that although there were many real Dark Lords of the Sith, many of them were self-proclaimed and never actually followed the traditions of the Sith, such as overthrowing the previous Dark Lord of the Sith. The name calling varied, but some of these Sith were called Pretenders. These Sith were known as false or pretenders because they claimed the power of the Sith on their own, rather than by tradition, which, in the eyes of real Sith followers and prophecies, made them false. Originally, the Sith were a group of Dark Jedi that crash landed on Ziost and integrated with the Sith species, which led to the name "Sith."
The Sith Order grew and grew, until it was believed to have been brought down following the New Sith Wars, with the fall of the Brotherhood of Darkness. However, the Sith Lord Darth Bane survived with his apprentice, Darth Zannah, which was the beginning of the Rule of Two. It was actually Darth Revan's holocron that taught Bane the theory, which led to many years of secretive planning by the Order of the Sith Lords, which eventually led to the formation of the First Galactic Empire, by Darth Sidious and Darth Vader. When they were destroyed, they were the last of the real Sith Lords, and any who followed were in fact referred to as pretenders.