The Jedi are characters in the Star Wars universe and the series' main protagonists. They use a quasi-telekinetic power, known as the Force, and weapons, known as lightsaber, which emits a controlled energy flow in the shape of a sword, to serve and protect their Republic and the whole galaxy from conflict or government instability. They sometimes moderate peace negotiations between planet and, if necessary, use their formidable fighting skills to quickly end unrest. The Jedi are led by a Council of 12 members consisting of the most powerful and wise members of the order. The Jedi are bound to a code of morality and justice and are trained in the use of the light side of the force but not the Dark side.
The Jedi are first introduced in the 1977 film Star Wars Episode IV: A New Hope as an order of warrior monks who serve as "the guardians of peace and justice in the galaxy" and embrace the mystical Force. Obi-Wan Kenobi (Alec Guinness) explains that the Galactic Empire had all but exterminated the Jedi some twenty years before the events of the film, and seeks to train Luke Skywalker (Mark Hamill) to be the Order's last hope. Darth Vader (David Prowse/James Earl Jones) is also established as the Jedi's main enemy. By the end of the film, Luke is on the path to becoming a Jedi. In the sequel, The Empire Strikes Back, Luke receives extensive Jedi training from the elderly Jedi Master Yoda (Frank Oz), even as he learns that Vader is in fact his father, former Jedi Anakin Skywalker. The third film in the original trilogy, Return of the Jedi, ends with Luke redeeming Vader and helping to destroy the Empire, thus fulfilling his destiny as a Jedi.
The prequel films depict the Jedi as troubled by the resurgence of the dark side of the Force and determined to fight their mortal enemies, the Sith. In Star Wars Episode I: The Phantom Menace (1999), Jedi Master Qui-Gon Jinn (Liam Neeson) discovers the young Anakin Skywalker (Jake Lloyd), whom he believes to be the "Chosen One" of Jedi prophecy who is destined to bring balance to the Force; the boy is eventually paired with Qui-Gon's apprentice, the young Obi-Wan Kenobi (Ewan McGregor), who promises to train him. The sequel, Attack of the Clones, establishes that the Jedi forswear all emotional attachments, including romantic love, which proves problematic when the now-adult Anakin (Hayden Christensen) falls in love with Padmé Amidala (Natalie Portman). In Revenge of the Sith, Palpatine (Ian McDiarmid), who is really the Sith Lord Darth Sidious, manipulates Anakin's love for Padmé in order to turn him to the dark side and become his apprentice, Darth Vader. Once corrupted, Vader helps Palpatine hunt down and destroy nearly all of the Jedi.
The Jedi's history before and after the time-line of the films is established within several novels, comic books and video games in the Expanded Universe of Star Wars media.
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Official Site: Lucas Films
Links: Wikipedia Entry The Jedi Encyclopedia The Jedi Order