The Sith used to rule the galaxy before the Republic and the Jedi Order overthrew them.
This is what the Sith are taking revenge for in Star Wars: Revenge of the Sith.
As you could hear it from Master Windu and Palpatine's dialogue.
MACE WlNDU: The oppression of the Sith will never return. Your plot to regain control of the Republic is over . . .
It is related to the Star Wars' Expanded Universe. Long story short:
There was only Jedi Order
A group of radical Jedi wants to use negative emotion such as anger, envy, and selfishness as a source of the force
This group of radical Jedi was kicked out and exiled onto a lonely planet outside of the Old Republic
They started calling themselves Sith and ...