Dark Lords are the rulers of the Sith. (The Sith are a race of beings, remember. See descriptions.) There may be lesser Lords, but the head of each of the three Sith clans is the Dark Lord. These three Dark Lords, the heads of the three clans (Warriors, Sorcerers, and Armorers/Weaponry) rule their people... although now the Warrior clan has tentatively pulled away from triumvirate rule. Nothing official yet, though.

The dark jedi (aka Palpatine, Vader, those types) are called Darths. NOT Dark Lords.

This universe is a little bit different (in semantics) from the George Lucas one. I just want to state this to alleviate any possible confusion.

Thank you~