Hm...the way I understood was that Korriban was one of the outposts of the Sith Empire, and had been forgotten by the "true Sith" along with Malachor 5. You also asked what "real Sith". I am not sure what a real Sith is, but they were the people who were settled on Korriban when the first renegade jedi (Naga Sadow and company, the way I understood KOTOR 1) arrived. According to the subtitles on KOTOR 1 loading screens, the true Sith were thought to have died out long before the events in KOTOR 1. However, judging from the things we learn in KOTOR 2 , it is apparent thay only the Sith on that one outpost planet died out, leaving an empire behind somewhere. This is, at least if you chose LS Revan, what Revan went to fight after the events of KOTOR 1, deep in the Unknown Regions.
As I said, these are my opinions, I could be wrong. Feel free to correct me if I am.