The Sith are the Sith. They are apparently (depending on what's still canon) an ancient order of Dark Side users with a specific philosophy and a policy (at least by the prequel period) of one master, one apprentice, in a combative relationship. read more
Kylo and the Sith have very little difference, other than skill and practice, the Knights of Ren are basically a cult in service of whoever they perceive to be a god (in this case Kylo and Vader), that is why Kylo is specifically the leader. read more
All we know about them is that they are a dark side organization that includes Kylo Ren and at least six other members. Really, the only similarity between them and the Sith is that they're both dark side organizations. It's not even certain that all of the Knights of Ren even possess force powers. read more