- First comes the day
Then comes the night.
After the darkness
Shines through the light.
The difference, they say,
Is only made right
By the resolving of gray
Through refined Jedi sight." - ―Journal of the Whills, 7:477[src]
The Jedi Order was an ancient monastic peacekeeping organization unified by its belief in and observance of the Force, specifically the light side. The Jedi were guardians of peace and justice in the Galactic Republic for thousands of years before the Clone Wars. While well known as diplomats, the Jedi carried lightsabers, the symbol and weapon of choice of the Order. Led by the Jedi High Council, Jedi Masters would instruct young Padawans on the ways of the Force and the Jedi Code, training them until they had achieved full status as Jedi Knights. Centered in the Jedi Temple on Coruscant, the Jedi used the Temple as a base of operations from which they would be sent out on missions, or stay to study and learn from the vast Jedi Archives.
Thousands of years before the Battle of Yavin, the Jedi faced wars against their mortal enemy, the Sith, who they believed to have defeated. The Jedi Order thereafter served as a strictly peacekeeping organization, maintaining law and order during a thousand years of peace under the rule of the Galactic Senate. During the Battle of Geonosis, however, the Jedi took on the responsibility of the Republic army, and became military leaders to fight the Confederacy of Independent Systems in the Clone Wars. Unbeknownst to them, the conflict had been engineered in order to destroy them by Supreme Chancellor Sheev Palpatine, who was secretly the Sith Lord Darth Sidious.
(source Star Wars wika)