If Kreia's speculation is correct, Revan went about studying the ways of the Sith, named himself "Darth", and tried to forge an empire out of the Republic because he became aware of these "unknown" Sith and believed them to be a threat to the fate of the galaxy. Of course, being twisted by the Dark Side, we can't be certain about the truth of such implications, but it's out there, and if true, likely makes these True Sith a lot more bad*** than anything we've previously seen.
My personal belief is that they are the descendants of those who built the tombs on Korriban who in turn are of the Sith species, or descendants of the Sith species...and while retaining a number of physical characteristics of the Sith species to mark them as "different," appear largely human as a result of a couple thousand years of interbreeding.
Culturally and philosophically they therefore would fall closest to the Marka Ragnos/Ludo Kressh/Naga Sadow line of Sith and Dark Side Force users. My impression, however, differs from others stated. I would classify these Sith more along the lines of "fanatical" than cold, calculating, and heartless. The Sith whom the exiled Jedi encountered treated, maybe even worshipped, those Jedi as gods. I'm not versed enough in the EU to know whether that's how they viewed Ragnos, Kressh, and Sadow, but by virtue of the tombs on Korriban it would seem that the Dark Lords of the Sith were still revered as entities of great power at that time.
So in my mind, I'm thinking that rather than typical political and military channels, espionage, and treachery, the True Sith are believers in more abstract causes as a means to power, like "divine right" and the "glory of the Dark Side" and other similar concepts put forward by religious fanatics for why they must conquer the world. I can imagine that rather than just follow the tennets of the Dark Side philosophy like the line of Sith Revan spawned, they might actually "worship" the Dark Side in a manner of speaking.