Originally posted by Vensai
I never really got the difference between Sith and dark Jedi since Sith were originally Jedi.
Any Jedi who turns to the dark side is called a Dark Jedi. (As well as, occasionally and more generically, people who were taught the dark side to begin with. Like, say, the members of the Shadow Academy, who were simply taught by a Dark Jedi teacher).
A Sith is someone who follows the teachings that comes from one of the lineages that came from the Hundred Year Darkness/Second Schism Dark Jedi, i.e. the first Dark Lords of the Sith.
Additionally, some lineages may have additional qualifications. The Rule of Two requires you to not only have some training, but either be acknowledged by the current Master as an apprentice, or be able to kill one of the current Sith and seize the role (those who've yet to do so may be considered Dark Jedi [like Ventress], but not yet Sith). In general whenever there's a debate between two claimants of any order over whether one or the other is a true sith, the one who's not dead at the end is considered right, regardless of other factors.