Star Wars: Dawn of Darkness
A Dark Jedi is a term used to describe either a Jedi Knight who has fallen from the light and now embraced the Dark Side of the Force, or a Force user who has never followed the path of the light and simply draws strength from the Dark Side.
The reason the term Dark Jedi is applied to individuals who were never a Jedi is because of a common misconseption that only Jedi are Force users. In truth, there are countless races and organizations of Force users throughout the galaxy. None however are as prominent or numerous as the Jedi.
Dark Jedi can be of any race or gender, and can wield a wide array of weaponry made more effective by their Force powers.