Protectors of the Republic and enemies of Jango and Boba
The Jedi were once a flourishing Order that had served the Republic for millienia as it's protectors. However, the betrayal of Anakin Skywalker and Chancellor Palpatine, nearly led to the extinction of all Jedi. After Anakin was transformed into the wretched Sith-cyborg Darth Vader following an intense duel between him and his former mentor and friend, Obi-Wan Kenobi, the former hero of the Republic continued to carry out his personal vendetta on all remaining Jedi.
Darth Vader's mission to destroy all Jedi led to them being significantly reduced in number. Those who still existed had either moved far beyond the Empires grasp or abandoned their old lives as the Galaxies defenders. Luckily, though, the Jedi were saved from total annihiliation thanks to the birth of Anakin's twin children from his marriage to Senator Padme' Amidala. Luke Skywalker who was raised on the desert planet of Tatooine by Anakin's half-brother Owen Lars and his wife Beru came to be trained in the ways of the force after his aunt and uncle were murdered by the Empires Sandtroopers. With the help of "Ben" Kenobi and later Yoda, Luke was able to become a Jedi like his father before him and eventually help Vader in fulfilling his destiny as the Chosen One.
The Jedi and the Fett's rivalship stretches far back to when Jango Fett was mandalore of a group of Mandalorians. The Jedi group led by a young Count Dooku, were responsible for killing off all of Jango's comrades save for himself. Many years later, the Jedi would once again be responsible for the death of someone close to a Fett, when Jango was killed at the hands of Jedi Master, Mace Windu. This latter event led to a bitter hatred towards most Jedi particularly Mace and Obi-Wan Kenobi.