Little is known of the mysterious order referred to as the Sith, a group shrouded in many layers of secrecy and deception. What is known is that like the Jedi order, they believe in a mystical bioenergy field called the "Force", which is apparently dualistic in nature. The Sith claim to draw their power from the "dark side" of said Force, which is fed by strong negative emotion.
What little history of their home galaxy has been revealed to us suggests that the Sith have been engaged in many campaigns against the Galactic Republic and the Jedi order, often using other factions at a remove, such as the Mandalorians and, more recently, the Separatists.
They do not seem to have very many mechs at their disposal compared to other known factions. Whether this is due to a reliance on their Force or a desire not to have traceable links to known manufacturers of battle machines is unknown. However, their use of the Force gives them a considerable combat advantage, making use of mechs potentially unnecessary.