Jedi were monks who studied the mystical powers of the Force. They studied only the Light Side of the Force. The Fallen Jedi, or Sith, studied the Dark Side Of The Force. The prime weapon for a Jedi is one's own telekinesis, mind power, and most of all, their Lightsaber. The Jedi fought for Peace and Justice for the Galactic Republic. But as an undercover Sith Lord named Darth Sideous, rose in power over the Republic, executed order 66; an order for the main military (The Clone Troopers) to execute any living Jedi within it's sight. The Jedi hardly survived. The most powerful Sith Lord's son, Luke Skywalker, survived the order as he was young and well hidden. He would later go to train other Jedi. Galen Marek was the son of one of the last known Jedi, Kento Marek. He was trained as a Sith, but later chose to rebel against the Empire (the form of Galactic Government that destroyed the Republic). Galen Marek, or Starkiller, was one of the most powerful Jedi, but was killed.
Originally formed as a religious group on the planet Tython, Jedi became revered as guardians of peace and justice in the galaxy. As mystical wielders of the Force and of their signature lightsabers, their powers inspired all citizens in the galaxy. The calm, considered demeanor of the Jedi made them ideal brokers of peace in times of conflict or dispute. Yet, for all their power and diversity, the Jedi were few. Often beset by foes in times of doubt and confusion, undercurrents of evil often challenged their order and the establishment they served, the most notable being the Sith. These dark warriors were the antithesis of the Jedi, their sworn enemies, and the battle between them brought the galaxy to war more than once. In times of crisis, the Sith could even use the dark side of the Force to diminish the powers of the Jedi, and prevent them from sensing the future.
Becoming a Jedi required the most profound commitment and astute mind, the life of a Jedi was one of sacrifice. To hinder transgression, those who showed an aptitude for the Force were taken directly from birth (or soon afterward) to train in the Jedi Temple headquarters on Coruscant or at smaller Jedi Enclaves as Padawans. From the beginning of their training, a Jedi was expected to adhere to a strict Code that included concepts such as rational thought, patience, and benevolence. Uncomfortable emotions such as hate, anger, and fear were thought to be destructive and lead to the dark side, so such things were banned from Jedi practice.
Children being trained as Jedi were called younglings and instructed in "clans" by a venerable, experienced Jedi Master, learning the ways of the Jedi and the powers of the Force. After Ruusan it had become normal that the Jedi trainees were always children. Once Yavin 4 had been established as Jedi headquarters, the Jedi initiates, which were no longer children, were trained first under the direction of Jedi Master Luke Skywalker, and, in time, under another Master's direction in groups. When the New Jedi Order began, the younglings were again trained in clans from childhood.