It’s a common, and somewhat painful, misconception that the Sith and the Dark Jedi are the same thing. While there are many Dark Jedi that follow Sith guidelines and beliefs, it doesn’t make them Sith by any means. “Dark Jedi” is a broad sweeping term that embraces all dark practitioners, while “Sith” specifically refers to a definite ideology.
Dark Jedi don’t even have to be particularly evil. Sometimes, a Dark Jedi is just a Jedi who has given up on adhering to the Light Side of the Force. Even occasionally, a Dark Jedi refers to somebody who hasn’t gotten any formal Jedi training, but somehow began careers under another Dark Jedi. And even rarer, but still occurring, is the subset of the group of Dark Jedi who aren’t Force sensitives at all, but they serve the Dark Side.
More generally speaking, a Dark Jedi is a fallen Jedi. It’s the most common usage. A Jedi who has taken to the Dark Side and embraced it.
It’s pretty simple.
But then we get to the Sith, and things get more than a little bit complicated. The term “Sith” refers to at least 14 different factions. Including the original Sith (not to be confused with True Sith), the species. The Sith as a species were a little bit timid and meek, and were enslaved by the Dark Jedi. Through Sith Alchemy, the Sith mated with the Dark Jedi and formed a kind of entirely new species.
No matter which way you look at it though, all 14 or so of these different Sith-name-holding groups adhered to the Dark Side. They had a belief system in place, just like the Jedi. They are, for all intents and purposes, the opposites of the Jedi. They all, these Sith, have the same ideals and powerful abilities. The differences mainly come in the rankings and the leaders of the time.
After the rise and fall of the interbred original Sith and Dark Jedi species, the name Sith stopped referring to species at all and turned into an identification for those who were dedicated to the ancient Sith philosophy.
Sith, unlike Dark Jedi, have no real use for Lightsabers. They use them in a way to mock the Jedi. But their powers are generally such that they could overcome any opponent without the use of one. Both the Dark Jedi and the Sith are prone to using artificially made red crystals in their lightsabers, but the Dark Jedi have also been seen using the more traditional colors.
The Sith also had a habit of using Dark Jedi for their own gains. Seducing them and persuading them to do the dirty work, while never really sharing the Sith secrets. Luckily for the Sith, the Dark Jedi were glad to serve for the most part and saw it as a way to expand their knowledge of the force, even without the Sith techniques being taught to them.
The Sith that people are most aware of are the Sith from the New Sith Empire. This is some time after the Sith Lords started adding “Darth” to their new Sith names, and we start seeing the likes of Count Dooku (Darth Tyranus), Emperor Palpatine (Darth Sidious) and Anakin Skywalker (Darth Vader). The Sith line, however, is long and vast and holds names that perhaps people wouldn’t expect to see, including Jacen Solo (Darth Caedus) – the son of Han Solo and Princess Leia Organa.