Sith Lord is the title carried by former Grand Masters as former Dark Lords of the Sith. Members the Grand Master deems worthy can also be elevated to the status of Sith Lord, this is often the case with Deputy Grand Masters after completing their formal apprenticeship as a Shadow Hand.
Sith Lord is the middle rank of the Order of Sith Lords and is considered a full member, as oppossed to the Dark Lord of the Sith as the Master and Shadow Hands as apprentices.
History
While obviously referring to a master of Sith knowledge, the title of 'Sith Lord' (or 'Sith Lady' if referring to a female Dark Lord) seemed to be used somewhat ambiguously to refer to various things, including all Sith, the Dark Lords of the Sith, Sith Masters in Darth Bane's reformed order, or as a rank akin to that of the Jedi Master for the Jedi Order. Several Sith Lords also held the title Darth.
Originally, "Sith Lord" referred to a high-ranking member of the Sith race's society. As the Sith civilization progressed, many claimed the title; Adas was one such lord.
List of Sith Lords
Alphabetical; The Dark Lords of the Sith (leader of the Sith or Sith Master) are shown in bold.
Ancient Sith
Reformed Order