The first Star Wars film introduced two major schools of the Force: the Jedi, defenders of good who used the light side of the Force, and the Sith, evil beings who used the dark side to serve selfish purposes.
But the Force is bigger than just Jedi and Sith. It "binds the galaxy together," as Obi-Wan says, and so it makes sense that other beings affected by the Force have developed their own philosophies and traditions. The Expanded Universe focuses on the struggle between Jedi and Sith, but still makes time to explore alternate Force organizations.
Check out these different Force traditions for an explanation of their history and beliefs.