The Jedi Order, later known as the Old Jedi Order and referred to as the Holy Order of the Jedi Knights, was an ancient monastic peacekeeping organization unified by its belief and observance of the Force, specifically the light side. The Jedi were the guardians of peace and justice in the Galactic Republic, and became the most famous of the various groups that relied on the Force for their power. Led by a series of Jedi Councils, the Order grew over the millennia despite facing many trials, most often threats from the Sith, wielders of the dark side of the Force.
During several Great Schisms, Dark Jedi broke away from the order. These schisms led to several wars including the Hundred Year Darkness after which the Dark Jedi fled and founded the Sith Empire. The Sith were eventually found by the Republic and their discovery led to The Great Hyperspace War, and a Sith invasion of Coruscant. The Jedi on Coruscant, led by Jedi Guardian Anavus Svag and Jedi Master Memit Nadill, successfully defended the planet and defeated the Sith. But their dark brethren wouldn't stay defeated for long and during the next several thousand years many wars would be fought. A thousand years later, after The Great Sith War, the Mandalorians saw their chance and attacked the weakened Republic. After the war, the Jedi that had led the Republic's military, Revan and Alek Squinquargesimus, disappeared. When they returned it became clear they had turned to the Dark Side, and during the war that followed many Jedi died. During the aftermath the Order was left numbering less than one-hundred Jedi, until it was rebuilt by the Jedi Exile and the Lost Jedi.
Many years later the galaxy was again engulfed in a War between the Republic and the Sith. Led by Lord Hoth, the Jedi were able to regroup and push back the Sith, who eventually withdrew to the planet Ruusan. After seven long battles on Ruusan the Sith were defeated and presumed wiped out. One of them managed to survive and reorganize the Sith Order. Though Darth Bane and the Sith Lords that followed in his footsteps remained hidden, they were patient. Millennia later they revealed themselves, determined to have their revenge. After a crisis, the Republic was split in two and a war once again engulfed the galaxy. During the Clone Wars, the Republic's armies were once again led by the Jedi. During the first battle of the war, nearly two hundred Jedi died before Master Yoda and a Clone army arrived to assist them. However, nobody expected that the Supreme Chancellor was in fact a Sith Lord. The Jedi eventually learned the truth, but it was already too late. The Order was virtually annihilated by the Sith and betrayed by the Republic they had served for millennia. Only a few survived the Great Jedi Purge. However, Anakin Skywalker, with an indirect help of his son, Jedi Luke Skywalker eventually defeated the Sith and Luke formed the New Jedi Order during the time of the New Republic.