The Jedi Knights were an order of warrior priests that followed the "light" side of "The Force". Their members progressed through three levels of rank: first the Padawan Apprentice who begins training under a Jedi Knight at a young age, about 12, then Jedi Knight, and finally the level of Jedi Master. Eventually they could become a member of the Jedi Council which acts a governing body for the order and an advisor board for the Republican Senate.
Among other tasks, the Jedi were responsible for law enforcement and peacekeeping in the Galactic Republic[?] (though they were considered neither police nor an army.) To that end, they use The Force and extensive training to become extremely formidible in battle. Their traditional weapon was the lightsaber which in the hands of a skilled wielder, could be more than a match for most opponents even when they are armed with ranged weapons. A common task for the Jedi is to construct their own lightsaber.
They were almost completely wiped out in the course of the "clone war" through the actions of followers of the "dark side", led by the Sith lord Darth Sidious. All Jedi, save two, were wiped out following the clone war by Anakin Skywalker, who had by this time succumbed to the dark side of the force and was then known as Darth Vader. The last remaining Jedi were Obi Wan Kenobi, Anakin Skywalker's own teacher, and Yoda.
"The Force" is described as being an energy field that is generated by all living things and permeates the universe and all the things in it. Its principles resonate with those of some real world religions like the shinto religion of Japan and certain celtic, druidic concepts. As explained in the movie Star Wars: Episode I, Jedi get their ability to manipulate the Force from a symbiotic organism in their body called midichlorians (an obvious play on mitochondria.) The higher the midichlorian count in the blood, the stronger the Force is with the particular individual. Anakin Skywalker had a reading of over 20000 which was even higher than master Yoda's.
This power does not come without a price. The power comes comes with great temptation to misuse it, and emotions such as hate can cause the Jedi to be overcome by the "dark side" of the force. The dark side of the force greatly amplifies its power in the individual, but also is taxing to the body, as is evidenced by Palpatine's appearance by the time of Return of the Jedi. When one goes over to the "dark side", there is little hope of recovery.
The actions of Luke Skywalker and his relationship with the Force were a principle plot device in the films. In the novels set after the civil war, Luke's main task is to reestablish the Jedi order by training a new generation of Jedi Knights.
Major Jedi include:
New Zealand and the UK due to a grassroots movement which urged people to declare their religious affiliation as such in order to protest the inclusion of religion on the census at all. In Australia more than 70,000 people declared themselves members of the Jedi in the 2001 census.
Jedi is still some way from being recognised as an "official" religion, however.