From The Old Republic Wiki

The Force is an all encompassing power that binds every living being in the galaxy together.

The Force is divided into two distinct parts, The Lightside, and The Darkside. Both sides of the force rely on different believes. The ability to manipulate the force is a rare ability. Those who can manipulate the force are called Force Sensitive. Generally, the Lightside and the Darkside of the force are associated with two distinct factions, the Jedi and the Sith.

Both factions are usually at ends with each other, due to opposing belief's on the usage of the force.