Originally Posted by Doctorjones
Does that mean that Count Dooku and Anakin Skywalker are dark Jedi. Because they were both once Jedi, and turned to the Darkside.
Yes, they are Dark Jedi, AND they are Sith.
As pointed out, the Sith are a specific Order of Dark Side users. A "Dark Jedi" would (logically) be any Jedi who turns to the Dark Side.
In the movies there are only two kinds of force users ever mentioned... Jedi (who are not supposed to use the Dark Side) and the Sith (who make it a point to use the Dark Side).
The EU writers were (at one point in time, on again off again apparently) forbidden to use the term "Sith" in their works before the Prequels since Lucas wanted that territory for himself. So they came up with the term "Dark Jedi" basically to describe ANY Dark Side user. Then later they made up that the Sith were these red aliens conquered by Dark Jedi in the past and then the Dark Jedi leaders came to be called Sith Lords etc (this was before the term "Darth" was revealed as a Sith title, in TPM by Lucas... before that it was assumed to be a personal name for Vader, since Obi-Wan even calls him "Darth" in ANH!). A lot of stuff wasn't known until the prequels. The Star Wars: Behind the Magic CD set repeats the popular wisdom that Vader was the only Dark Lord of the Sith (that there was only ONE Dark Lord of the Sith at a time). We knew as of ESB that the Emperor had the Force, but not that he was a Sith Lord himself and that there were only ever two Sith at a time.
So now in the EU we end up with dozens or hundreds of people who practice the Dark Side but aren't Sith. And even those who call themselves Sith but really aren't (they even made a point of mentioning this with regard to Assaj Ventress in the Clone Wars, where Dooku ridicules her for calling herself Sith).
And finally Dark Side users that are neither "Dark Jedi" nor Sith. And they give us "Light Side" (the term never appears in the movies) users who are not Jedi. So it runs the spectrum, so I don't blame you for being confused.
Anyway, in the movies we have Maul & Sideous who are Sith, but not Dark Jedi. Dooku and Vader who are former Jedi turned Sith, so the term Dark Jedi could be applied to them. And the rest of the characters who use the Force are simply Jedi (we assume that Leia will be trained as the first Jedi pupil of Luke Skywalker, by the end of ROTJ).
In the movies there has never been a Dark Jedi who didn't also become a Sith. But in the EU at least, it would easily be possible for such a creature to exist. Just have a Jedi fall to the Dark Side but not join the Sith Order. Since the Sith Order is effectively extinct after ROTJ, there really aren't anymore Sith (but a few pretenders to the throne I suppose). The Sith line might be able to be restarted, but their line was broken. The Jedi Order line still continues in Luke, trained by the last Jedi, Yoda.
Sorry for the wordy digression, I just wanted to help clear that up.