Ryan McAvoy said:

aren't 'Sith' and 'Dark Jedi' basically the exact same thing???

To me (as I did not read every novels nor comic books) the Sith are like an ancient religion. Once there are no more Sith, you can't have more Sith. Unless someone learns to use the Force from a Sith holocron, directly from a dark lord of the Sith.

So, to me you can have: Sith Lords, or Dark Jedi (a Jedi who turns evil but without the help of a Sith Lord) or a random guy that knows the ways of the Force and do bad things with it without being a Jedi nor a Sith.

So yeah... they are more or less the "same thing" in the sens that they are all people using the Force for doing bad things or for their own interest. But as a matter of plot they are all a bit different.