One of the prominent elements of Star Wars is the Force, an omnipresent energy that can be harnessed by those with that ability, known as Force-sensitives. It is described in the first produced film as an energy field created by all living things surrounds us, penetrates us, binds the galaxy together. The Force allows users to perform various supernatural feats and can amplify certain physical traits, such as speed and reflexes; these abilities vary between characters and can be improved through training. While the Force can be used for good, it has a dark side that, when pursued, imbues users with hatred, aggression, and malevolence.
The six films feature the Jedi, who use the Force for good, and the Sith, who use the dark side for evil in an attempt to take over the galaxy. In the Star Wars expanded universe, many dark side users are Dark Jedi rather than Sith, mainly because of the Rule of Two. (source : wikipedia)