The Force is an energy that resides throughout the galaxy. Those who can sense it are called Force-sensitives and they can use the Force to perform amazing feats.
Throughout the history of the galaxy, philosophies on the nature of the Force have been debated and even warred over. As such, Force practitioners have adhered to either the Light Side or the Dark Side of the Force. There is a balance of the Force, and both the Light and Dark sides believe they are maintaining that balance through their actions.
Currently, the Dark Side reigns over the galaxy under the rule of the Empire and the Sith Lords, Emperor Palpatine and Darth Vader. The Light Side is the enemy with the Jedi Order fallen and any Jedi survivors of the purge forced into hiding. Any other Force-sensitives found by the Empire are turned to the Dark Side or destroyed.