The Jedi are Force-sensitive members of the Jedi Order; who harness the power of the Light-side of the Force and follow a strict code of conduct, vowing to protect the galaxy. Their weapon of choice is the lightsaber.
Force-sensitive individuals are usually selected at a young age to train to become Jedi. A Jedi apprentice is known as a padawan, while those who have completed training are known as Jedi Knights; and the higher members of the Order are known as Jedi Masters.
When Emperor Palpatine assumed power, he ordered the extermination of the Jedi. The only two known survivors of the massacre were Obi-Wan Kenobi, who went into exile on Tatooine; and Yoda, who went into exile on Dagobah. Both later acted as mentors to Luke Skywalker, who completed his training following their deaths and became the last Jedi in existence.