What is the difference between dark jedi and sith

The Dark Jedi existed before the Sith Lords. The name Dark Jedi (or Fallen Jedi depending on whether or not they originally sided with the light side) refers Jedi that either rejected light-side teachings in favor of the dark side or Jedi that were taught by Dark Jedi and thus became dark themselves. After a full-on Galactic war called the Hundred-Year Darkness between the Dark Jedi and the Jedi Order the Dark Jedi were banished to a planet called Korriban. This planet is the homeworld of an actual race of beings called the Sith. The Sith took in the Dark Jedi and over time they inter-mated with each other and created the modern encarnation of the dark side the Sith Lords. In time the term Sith replaced the term Dark Jedi to describe powerful dark side users because of their lineage leading back to the Sith people. Because these inter-bread beings spread throughout the Galaxy teaching the dark side to force sensitives it is now possible for someone to be a Sith and not be descended from them (i.e. Darth Sidious, Vader, & Maul). But Darth Sidious did bring back the term Dark Jedi as a rank for force sensitives in his military in the Second Imperium.