Definition: A Jedi is a member of the Jedi Order, an organization of Force users who use the light side of the Force. In contrast to the dark side, which is associated with selfishness, anger, and violence, the light side of the Force is associated with positive traits and emotions, such as compassion and healing. While they are not the only organization of light-side users in the Star Wars universe, the Jedi are probably the oldest and most well-known.
Origins of the Jedi Order
The Jedi Order began with the discovery of the light side of the Force around 26,000 BBY. Over the next several hundred years, the Jedi grew into a philosophical order and then into defenders of the fledgling Republic. They first encountered the Sith, an order of dark side users, during the Great Hyperspace War around 5,000 BBY.
The Sith would remain the primary enemy of the Jedi for the remainder of Star Wars history, and the draw of the dark side would always be powerful. Those Sith Lords that the Jedi Order faced were often fallen Jedi, from such early Sith as Freedon Nadd around 4,400 BBY (in Tales of the Jedi) and Exar Kun around 4,000 BBY (in Knights of the Old Republic), to Anakin Skywalker (Darth Vader) in the Star Wars films.
Jedi in the Star Wars Films
The organization of the Jedi Order changed several times over the course of the Expanded Universe, although it was more stable than the Sith Order. By the time of The Phantom Menace, the Jedi were cemented in their role as peacekeepers for the Republic. They had renounced attachments such as marriage, and increased their numbers by seeking out young Force-sensitive children and raising them in the Jedi Temple. With training, these Younglings moved up the Jedi ranks to Padawan (apprentice), Knight (one who has passed the Jedi Trials), and Master (a Jedi exceptionally skilled in the Force).
In Revenge of the Sith, Anakin Skywalker, a prominent member of the Jedi Order, became a Sith and helped slaughter the Jedi. Very few Jedi survived the Great Jedi Purge, which began with Order 66. Among them were Yoda and Obi-Wan Kenobi, who trained Anakin's son Luke Skywalker in the way of the Jedi.
Changes in the Jedi Order
After Return of the Jedi, Luke rebuilt the Jedi Order, building a Jedi Academy on Yavin 4. Luke's New Jedi Order began with a group of only twelve students, gradually expanding to include Force-sensitive beings with various levels of training and experience.
Many of the changes in the Order's organization were based on practicality and necessity. For example, while Yoda rejected a nine-year-old Anakin Skywalker as "too old to begin the training," the New Jedi Order could not afford to be so picky. At their height before the Yuuzhan Vong Invasion, the Jedi Order only had about 100 members -- a pale shadow of their former state.
Many of Luke's students were adults whose Force-sensitivity had gone undetected during the time of the Empire. Some, like Leia Organa Solo, did not begin formal training until late in life. In addition, by allowing marriage and other attachments, the New Jedi Order did not command such strict and absolute loyalty from its members as the old Order.
Fate of the Jedi Order
The Jedi faced political tensions after the Second Galactic Civil War (chronicled in Legacy of the Force). In the ongoing series Fate of the Jedi, Chief of State Daala exiles Luke for failing to prevent the fall of Darth Caeudus, the Jedi Order is placed under surveillance, and a number of Jedi begin suffering from a mysterious violent psychosis.
The Jedi Order eventually recovered and continued to grow, but faced another Purge when Darth Krayt's Sith empire attacked the Jedi Temple around 130 ABY. The surviving Jedi went into hiding, and the struggle of Luke's descendent Cade Skywalker to destroy the Sith once again is chronicled in the comics Star Wars: Legacy.