A Dark Jedi is a Force-user who has succumbed to the Dark side of the Force, and was mostly affiliated with the Sith Empire, which was led by Darth Revan during the Jedi Civil War. More specifically the term referred to those who turned their back on the Jedi and gave into the Dark Side but did not serve the Sith.
Dark Jedi were often unaffiliated with either the Sith or the Jedi and used the dark side for personal gain and wealth rather than power like their Sith counterparts, one such example is Juhani who neither served the Sith Empire nor became a full Sith.
Dark Jedi are occasionally compared to Grey Jedi.
Known Dark Jedi
Jedi Civil War
During the Jedi Civil War Dark Jedi served the Sith Empire (Jedi Civil War) as leaders of its armies and agents sent to hunt those their Sith superiors ordered. It appeared that they were unlike those who received training on Korriban and after turning their backs on the Jedi they received no Sith training and went straight off to fight the Jedi.
After Darth Malak assumed control of the Sith Empire he blockaded the planet Taris and sent his apprentice, Darth Bandon, with a team of Sith Troopers and Dark Jedi to capture Bastila Shan aboard the Endar Spire.
After she managed to escape Taris alive Dark Malak dispatched squads of 3 Dark Jedi to capture or kill Bastila on Tatooine and Kashyyyk. Dark Jedi also encountered the amnesiac Revan in their base in Ahto City on planet Manaan. All teams of Dark Jedi were subsequently killed.
The current Sith Empire isn't known to utilize Dark Jedi, instead they prefer to train their force users to become full Sith. This is likely because they had the time before, during and after the war in which to train them, unlike the Sith Empire during the Jedi Civil War who needed them immediately to power and lead their war effort which resulted in many not actually receiving training in its New Sith Order.
Independent Dark Jedi were better known during the Great Galactic War, some even served the Republic. One such Dark Jedi was Dar'Nala who fell to the Dark Side whilst attempting to destabilize the Treaty of Coruscant.
Dark Jedi now during the Cold War are still not known to serve the Sith Empire.
The Jedi Knight and the Jedi Consular have the ability to become Dark Jedi should they fall away from the Councils teachings and give into the Dark Side, although they do remain loyal to the Republic and the Jedi Order.