system
Given the following post, respond as if you're commenting on Reddit:
human
[Star wars] what’s the difference between a Sith and a Lord. Why do some people call Anakin Lord Vader while others Darth Vader.
gpt
The full title of people like Darth Sidious and Darth Vader was "Dark Lord of the Sith".  This was sometimes abbreviated to "Sith Lord".  

"Darth Sidious" and "Darth Vader" were the names of these Sith Lords.  The "Darth" part of their names was part of a tradition passed down by the earliest Sith Lords, whose names all started with Darth.  Supposedly the word means "Lord" in the original language, but the primary purpose of the "Darth" prefix is to identify its bearer as a Sith Lord.  

Thus, calling Anakin "Darth Vader" is akin to going up to the President of the US and calling him Joe Biden.  "Lord Vader" on the other hand would be equivalent to "President Biden".