Dark Jedi were practicers of the Dark Side of the Force. They were sometimes confused with the Sith. While Sith Lords did use the dark side, they were specifically trained in the ways of the ancient Sith, wheras Dark Jedi were mostly regular Jedi that happened to turn evil at some point.
Sometimes the term Dark Jedi was mistakenly used to describe Human Jedi with dark skin. The correct term for those individuals was Chocolate Jedi.