Canon in Star Wars is pretty well maintained but there are varying degrees of it. They track it in a database called the Holocron and basically their are 5 levels:
G-canon is absolute canon; the movies (their most recent release), the scripts, the novelizations of the movies, the radio plays, and any statements by George Lucas himself. G-canon overrides the lower levels of canon when there is a contradiction. Within G-canon, many fans follow an unofficial progression of canonicity where the movies are the highest canon, followed by the scripts, the novelizations, and then the radio plays.
T-canon[1] refers to the canon level comprising only the two television shows: Star Wars: The Clone Wars and the Star Wars live-action TV series. Its precedence over C-Level canon was confirmed by Chee.[2]
C-canon is primarily composed of elements from the Expanded Universe including books, comics, and games bearing the label of Star Wars. Games and RPG sourcebooks are a special case; the stories and general background information are themselves fully C-canon, but the other elements such as character/item statistics and gameplay are, with few exceptions, N-canon.
S-canon is secondary canon; the story itself is considered non-continuity, but the non-contradicting elements are still a canon part of the Star Wars universe. This includes things like the online roleplaying game Star Wars: Galaxies and certain elements of a few N-canon stories.
N-canon is non-canon. "What-if" stories (such as stories published under the Star Wars: Infinities label), crossover appearances (such as the Star Wars character appearances in Soulcalibur IV), game statistics, and anything else directly contradicted by higher canon ends up here. N-canon is the only level that is not considered official canon by Lucasfilm.
Somebody once described the various levels of Star Wars cannon like looking at the SW universe through a telescoping lens of camera. At the wide angle you have a large view of possibilities that could happen but not much detail (ie you might tell that a speck on the frame is a person but you can't tell what they are wearing or even if they are male or female). As you zoom you in get more detail till you get the "real" story (ie you zoom and you can identify your friend who really did lose a bet and wore a clown costume to work). I actually find this analogy works well watching almost anything that is being adapted to a new media.
For the most part it works pretty well. Right now though I'm a little annoyed the Star Wars:Clone War Mandalorian storyline completely irreconcilably retconning what was established about Mandalore in the excellent Clone Trooper book series (which was being followed up the Imperial Trooper series but is being abandoned by the author due to these changes). Mandalorians were presented in a lot of ways up till now but never as pacifists - it ridiculous but they way it is now - ah well.