take it from a star wars geek:

the sith were originally an alien species outside the realm of the jurisdiction of the old republic. when the first dark jedi uprising occured roughly 20 millenia prior to the movies, the dark jedi were cast out, and stumbled upon the sith worlds. the sith hailed the dark jedi as gods, and so began the sith empire, where sith magic and the dark side of the force mingled to produce terrifying powers.
after the great hyperspace wars that toppled sith lord naga sadow's power base and exiled the dark lord to yavin 4, and the great sith war that ruined sith lord exar kun's bid for power, the sith tradition remained dormant until roughly 2000 years prior to the movies. the sith species had faded into extinction, so at this time the word sith took on its new meaning, that of a cult dedicated to the dark side. the new sith eventually destroyed each other in their bid for power. the 20 thousand survivors, united under sith lord kaan, established a brotherhood of darkness based on the axiom "rule by the strong". this army was driven back by the army of light under lord Hoth to the planet ruusan, on which seven titanic battles were fought. the sith lost all but two of the battles. kaan then came up with the idea of a "thought bomb", created entirely of the dark side of the force. when hoth and his army came upon the sith, kaan triggered the bomb. the implosion of the bomb sucked all of the spirits of the sith and jedi into a state of und...

there is a power called the force that every living thing poseses like every thing there are two sides to this the light side (jedis use:luke skywalker, obi-wan, yoda ect.) then there is the dark side(siths use vader,maul,Sidious ect)the sith do not see it as evil more of power, a weapon, some 'sith'dont see the force as light or dark but as jsut the force this is frowned upon by the jedi councle (they are closed minded fools)

Sith: the new Pope, take a look at him people the resembalence is creepy!

The evil version of the Jedi who has embraced the Dark Side of the Force. The Sith were suppressed for one-thousand years by the Jedi Order, until the first three Star Wars movies, when Palpatine/Sideous used deception to rise to power and nearly exterminate the Jedi with the help of Darth Vader, who was once known as Anakin Skywalker.

When someone acts so awesome that it's wicked.

1) A deceitful, promiscuous human female.
2) A pretentious, stuck-up bitch.
3) A woman in favor of murdering babies.

Despite what some idiots may think, a Sith is not a Dark Jedi. Known Sith lords include Darth Sidious, Darth Vader, Darth Maul, and Darth Tyrannus(Count Dooku).