For over a thousand generations an elite society of warriors known as the Jedi Knights acted as guardians to the Old Republic. They were warriors of light - protectors of freedom and justice. Jedi let the Force flow through them - performing amazing feats of unbelievable power and proportion.
The Old Republic flourished under their protection - until the power hungry Emperor Palpatine and a corrupt Jedi known as Darth Vader hunted down and eradicated the order of Jedi Knights. The only ones known to survive were Obi-Wan Kenobi and the Jedi Master Yoda. Thus began the era of the evil Empire.