The following represents a compilation of the various statements on the Star Wars canon policy, as stated by those who are in a position to speak on the subject, weighted by authority. A full list of references is below the main text.
In truth, it is best left up to the individual's point of view (Sansweet), but for a versus debate that isn't helpful. So, we must attempt to determine a set of hard rules based on what Lucas and his authorized agents say, and make sure that the way we determine what the rules are is the same for any other franchise's canon policy.
The real story of Star Wars is the absolute Canon of the films (Cerasi, Sansweet). The remaining parts of Canon are the film scripts, film novelisations, and the 1981 radio plays (Insider #23), with a canonicity that drops off in degrees (Sansweet) based on how close they are to the movies in format and content (Cerasi). The further you get from the movies, the more interpretation and speculation come into play (Cerasi).
Lucas and LFL have allowed other writers to expand upon his films with original fiction novels(Dice's 1st Insider quote), games, and comic books (StarWars.com EU intro). LucasBooks handles the editorial Continuity for these. The Continuity is an internal consistency -- ensuring the books don't contradict each other -- that LucasBooks decided to strive to maintain (Dice's 2nd Insider quote), with the express exception of works bearing the "Infinities"(*) logo (Cerasi). This is known as the Expanded Universe, and authors who write for it must know not only the Canon, but also the EU Continuity (Handley, Denning). This catalog of published works comprises a vast history with offshoots, tangents, and variants (Insider #23), though this history doesn't necessarily represent the true history of the Star Wars universe (Cerasi). It may serve as a window into the real story of Star Wars, and may contain nuggets of truth (Cerasi), but it cannot write the real story (Cerasi) nor is it clear whether those nuggets of truth come from the original fiction or simply the fiction's use of Canon. In any event, Lucas is the source of Star Wars, and isn't held to any of the EU stories (Sansweet, Community page).
There is much confusion among Star Wars fans as to what is true of the Star Wars universe beyond the Canon (Cerasi and others). Part of this is due to conflicting statements about what is and isn't the real story of Star Wars (see starwars.com EU intro (suggesting that Canon + EU = 'entire tale'), Rostoni in Insider ('everything's canon' except Infinities), Sansweet in the Encyclopedia ('Quasi-Canon'), the Star Wars Card Game's Rulebook that says the cards are canon, et cetera). Some use only the Cerasi/Insider Canon, some think it is all canon, and some, such as Mike Wong, think there are three levels, Canon, Quasi-Canon, and Official.
Recently, Lucas cleared up the messy issue by explaining that the books, games, and comic books are part of a parallel universe, another world created separately from his own (Lucas in Cinescape). By design, the settings in time of the parallel universe do not intrude on his movie time period, since LucasBooks disallows authors from playing with events and periods of time that Lucas intends to use. LucasBooks tries to use the events in the Canon universe in the parallel universe, but the fact that it is a parallel universe explains why Lucas isn't bound by it (as stated by Sansweet). In other words, events may happen similarly in parallel universes at the same time (for instance, the events of Episode IV are referenced in the Canon and the EU Continuity), but that does not mean that how the two universes ended up there is the same (for an example of such a concept, see the Star Trek episode "Mirror, Mirror"... Kirk is captain in both universes, but the way he achieved the captaincy was far, far different).
This also explains why Lucas does not allow the parallel universe to intrude on the time periods and events he wants to work with in his universe. If someone retold his tales, these events of another world might confuse Star Wars fans and violate the integrity of his work, and his vision for his universe. That is also why he works so closely with the authors who write the novelisations of his films (Cerasi).
So, the parallel Expanded Universe has a vast history (Insider), but we now know that it is not the real story of Star Wars (Lucas, implied in Cerasi, Sansweet 12-02, et cetera). Perhaps the offshoots, variants, and tangents (Insider) or any failures to maintain the internal Continuity (Dice 2nd Insider) are most easily explained away as additional parallels, but such a discussion is outside the scope of this document.
This does not mean that the Expanded Universe didn't happen. It did
happen, but it occurred in its own reality. If a fan prefers the parallel
Expanded Universe event over a Canon event, they can do so, since it only
means they are choosing one reality over another in the entire Saga of the
Star Wars universes (starwars.com EU intro, Lucas in "Splinter").
However,
the real story of Lucas' Star Wars universe is the Canon, and that must be
kept in mind by the fan or debater.
(*) Some claim that the use of the "Infinities" logo, used to explicitly place something outside the Continuity, proves that the Continuity has some measure of Canonicity, evidently through some form of whiplash. However, just because "Infinities" is specifically stated to be outside the Continuity doesn't make the Continuity slip any closer to Canon status.
These individuals usually also claim that, since a few EU elements have made their way into Canon cameos (such as the Outrider appearance in one of the Special Editions), this Canonicity is confirmed. Unfortunately, this is not the case. Several EU works bearing the "Infinities" logo have had elements of their stories make their way into EU Continuity events, but the "Infinities" stories are still not Canon, and are not still part of the Continuity. Therefore, to assume that something being used by a higher-canon-level story makes the entire lower level have higher-canon status is not only illogical, but also a *concept* unsupported in the Star Wars Canon Policy.
To see examples of "Infinities" elements in Continuity-level stories, take a look at this discussion on the topic.
It should also be noted that Warsie factions like to use a completely made-up policy ("logical extrapolation by us") in case of conflict between Canon and non-canon "official" material. They have crafted a criteria of contradiction wherein something non-canon must be specifically contradicted by the canon in order to be considered false, and that the falsehood only exists "on that point". For example, a numeral in the non-canon must be contradicted by a different numeral in the canon, or else they claim there is no contradiction, even if the situation is something like "a handful" versus "1.7 billion". The idea is perhaps best stated by Mike Wong (where he claims it is "Lucasfilms Licensing's official position"): "The movies are true Star Wars, but the books still count as a secondary source, whenever they are not directly contradicted by the movies (and contradictions can be harder to prove than you think; what seems like an irreconcilable contradiction to you might be solved in an eyeblink by somebody else, so don't get cocky)."
This concept not only has no basis in the Canon Policy, but it is a direct violation of the spirit of the Canon, and the Canon Policy.
References:
Cerasi, Chris. Quoted by Steve Sansweet on the StarWars.com site.
Denning, Troy. Interview with TheForce.Net
Handley, Rich. Post on Usenet's rec.arts.sf.starwars.misc. Text available, halfway down the page.
Lucas, George. Interview in Cinescape, July 2002 edition. Text available.
Lucas, George, writing in "Splinter of the Mind's Eye" by Alan Dean Foster, 1994 republication. Text available.
Rostoni, Sue. Interview in Star Wars Insider. Text available.
Sansweet, Steve. The Star Wars Encylopedia. Foreward.
Sansweet, Steve. "Star Wars: Community | Are we going to get more details about Boba Fett's past?". (Referred to as "Sansweet, Community" in the text)
Sansweet, Steve. "Star Wars: Community | Does Lucas consider Boba dead?" (Referred to as "Sansweet 12-02" in the text)
Star Wars Collectible Card Game Rulebook. Text available.
"Star Wars: Expanded Universe | Beyond the Films". (Referred to as StarWars.com EU intro in the text)
Star Wars Insider quotes, posted by Graeme Dice. Text available.
Star Wars Insider #23, interview. Text available.
Wong, Mike. "Sources".
More information, including quotes from several of the pages above,
can be found
here.
An excellent discussion between myself and "Cromag" on the topic occurs
here.
(By the way, you may want to be seated when you start to read it, because
Cromag actually demonstrates civility. If you're familiar with
almost any pro-Wars debaters, you should know that's highly uncommon, and
quite refreshing.)