There are a few variations. Most of it has to do with the distribution of power.
I can't and won't speak for anyone else, but I can talk about the First Order.
The Knights of Ren, the First Order's dark side users are group completely loyal to its Supreme Leader. They are lawful sorts, upholding the autocratic vision of the Galactic Empire. Ideologically, they are fellow travellers of the Sith, but they run themselves in a way that ensures that they don't squabble as much as their Sith cousins.
They also more so act as a elite sort of intelligence group, travelling around the Galaxy, doing their masters bidding. Probably butchered this, but [member="Castor Ren"] or [member="Darth Veles"] could probably give you a better run down.