|Planet of origin|
Tentacle beards, evil
- "I'm a Sith, you're a Sith, we're all Sith, and it's a Sith-tastic day here in the Sith Empire, with our Sith powers. You wanna Sith to the Sith with me? Sith, Sith, Sith."
- ―A Sith
The Sith were a species of red-skinned bipedal creatures from Korriban with a genetic predisposition to being powerful in the Force. They were all left-handed... why? Because they were sinister. (See what I did there?) Anyway, they resented the Sith cult for stealing their name, but it doesn't matter anymore because they're all dead.
Yes, there's a great deal of confusion as to what "Sith" actually means, and although some might think this is because George Lucas didn't properly plan things out, there's a perfectly logical explanation for everything that's... right outside... in my car... I can get it for you, if you just wait right here.
The species evolved on Korriban and basically stayed there for millennia fighting one another and doing evil things, until they were viciously attacked by the Infinite Empire, an even more evil power from space. The Rakata tried to enslave the Sith, but the Sith fought back, killed the invaders, stole their spaceships, and began a space empire of their own, including the planet Ziost. This was all thanks to King Adas, whose ferocity earned him the title Sith'ari, the Perfect Being and Overlord of all Sith. (Or at least, that's what all his statues and obelisks called him. Mind you, they were built by slaves under pain of torture and death. They may have exaggerated a bit.)
Later, some Human Dark Jedi showed up in Sith space, and the two races began having evil sex and interbreeding. Over the centuries, the Sith and Humans had produced so many hybrid offspring that pureblood Sith were difficult to find. Many powerful Sith Lords of the time were Sith/Human hybrids, including Marka Ragnos, Naga Sadow, and Ludo Kressh. Together, they formed a powerful Sith Empire that reached its peak under the leadership of Marka Ragnos, during the Golden Age of the Sith.
That all ended when the Sith initiated the Great Hyperspace War with the Republic. Sith fleets decimated Coruscant, but unfortunately, Naga Sadow fucked up his invasion and was forced to retreat. Republic forces retaliated by annihilating the Sith Empire entirely and nearly driving the Sith species to extinction. A dwindling number of Sith moved to Dromund Kaas, the new Home of Evil, because Korriban was too depressing and Ziost reminded everyone of Highlander 2. They were led by the Sith Emperor, who was also a Sith, but didn't look like a Sith. By this time, "Sith" could mean a person of the Sith species or a member of the Sith empire or a follower of the Dark Side, and the pureblood Sith were getting really pissed off.
Eventually, the pureblood Sith got so annoyed that anybody and everybody was being called "Sith" that they all just died of resentment. The Massassi were a genetic offshoot of the Sith, but they were just slaves and nobodies. A few of them survived on Yavin 4 for a while, then even they got bored and died.