In all of the Star Wars media, Sith are referred to as "Sith Lords", but each one of them is usually interchangeably called either a Lord or a Darth.
For example, this are some lines from the script of Revenge of the Sith:
GENERAL GRIEVOUS: Yes, Lord Sidious.
PALPATINE: (...) Do what must be done, Lord Vader. Do not hesitate.
DARTH SIDIOUS: You have done well, Viceroy. When my new apprentice, Darth Vader, arrives, he will take care of you.
And the same thing happens in other movies, books, comic books, etc.
What is the difference between these two titles, Darth and Lord? Are they the same thing, or are they ranks, like Jedi Knight and Jedi Master?