(The following includes spoilers for Harry Potter, The West Wing, Babylon 5, Farscape, Batman Beyond, The Dark Tower series, "Star Trek XI," and "The Lord of the Rings."  We've talked about this before, btw, but a refresher is rarely amiss.)

In the ebb and flow of interaction with our favorite fannish source material, fans inevitably run into spaces in the canon where there are no hard and fast answers, only clues.  Things that happen between episodes or books.  Off-screen antics implied but never actually stated.  Even actions and words that make it to the final print but can be interpreted in multiple ways.

If it happens on the series or on the page, it's canon.  If it happens on the screen or the page and is later retconned (retroactive continuity, or "we changed our minds later so it really happened this way") then it's annoying and convoluted canon, and can be ignored or not at your own discretion.

If the show's headrunner, or the episode's writer, or the book's author says, "This is what I intended for this to say," it's authorial intent.  Authorial intent is not canon.  No, really.  When J.K. Rowling announced after the publication of "Deathly Hallows" that Dumbledore was gay, that did not make it canon.  That made it authorial intent, subject to be accepted or not by the fans of the series.  Why is this?  Because authors change their damned minds.  Because networks hand down edicts.  Because actors get pregnant or arrested on DUIs or die.  *moment of silence for John Spencer*  Because cancellation threats mean wrapping up storylines faster than originally intended.  (Babylon 5 and Farscape fans are familiar with "We had to finish out the epic story HOW fast?" syndrome.)  Because some storylines fizzle and others appear that work better.  Because two characters intended to pair up at the beginning have more chemistry with different characters instead.  Because the audience complains.  Because the audience isn't the one desired.  Because the head writer got (or lost) religion.  Because the publishing company wants to turn the trilogy into a twelve-book deal and the original plot barely covered two books with padding.  Because the showrunner thought one thing and didn't relay it to the rest of the writing staff, who went in completely another direction, and when the showrunner states in interviews, "Obviously, ABC," the audience who tuned in to the actual series are all united in their confusion of, "But, but, did you not notice XYZ?"  Because the producers become angry at the actors or the audience or the show itself, or they get bored with the book series, or because they get hit by a car and add the experience into their epic fantasy series.  (Stephen King, I'm looking at you.)

Ideas change, and what the author says one day may not be what's in the final print tomorrow, and may be a whole different story next week.  That goes for what the actors say, too.  Even when they're on stage at a con saying, "Oh, I totally meant LMNOP when I moved my eyes like that."  Later scripts might declare that the character was in fact replaced by an evil android during that scene.  You never know.

Authorial intent can become fanon. 

When two or more fans agree that someone happened which wasn't shown, or that something which was shown happened for a particular reason that isn't flat-out stated in the canon, this is fanon.  Fanon is a shared fannish reality of a source material, be it something the writers said, or something the fans noticed or imagined or hoped.  Sometimes the fanon is closely related to the source.  For example, most "fade to black" scenes in a television show are intended to imply that the characters in question then went on to have sex, and in general, the fans of that show agree with one another (and the producers) that sex did in fact take place immediately after the cutaway.  However, if no sex was shown, and the characters in question did not mention it again, it is also possible fanon to assert that in fact they broke apart from their kiss, exchanged passionate handshakes, and then played Parcheesi for the rest of the unspecified time.  (Doctor Merlin suggests this is not the interpretation most suited to later character interactions in most cases, but remains possible.)

Multiple fanons exist.  One group of fans will swear that when these two characters are in a room together, their interactions are stilted and difficult because they hate each other.  Another group of fans will declare it Gospel truth that the difficulty is because the characters want to shag like bunnies.  A third group of fans will claim it's all because the actors in question were hungover at the time and off their game, and it's only the dialogue in the scene that matters.  A fourth group may see the dialogue as being a carefully-constructed set of mutual lies, based on the obvious truth that both characters were being mind-controlled at the time by aliens.  Fanon is fun.  Also headache-inducing, especially when confronting a True Believer on any particular piece of said fanon.

Your fanon is your fanon.  My fanon is my fanon.  And that's okay.  Parts of your fanon may be jossed by later canon.  Parts of my fanon may be jossed by later canon.  It happens.  Blaise Zabini is not a Caucasian girl.  Terry McGinnis is not Bruce Wayne's clone.  And so on.  Accept the new canon, go AU, pretend nothing exists past a certain point in the canon (a personal favorite tactic of mine), whatever gets you through your day.  Just remember that other fans are doing the same thing in their own ways, accepting or dismissing the new canon as needed for their own sanities. Going into their sandboxes to kick up a storm and whiz on their toys because their fanon is not your fanon is counterproductive and rude, and will only lead to wankstorms.

Fanon that is accepted by the majority of the fandom is still fanon.  If your fandom's fanon claims that members of Species X mate for life, good for you.  Unless it's in the canon, that's still fanon, no matter how widely-accepted the fanon might be.  If your fandom's fanon says a minor character has a particular name, until and unless the name makes it into canon (such as Nyota Uhura and Hikaru Sulu, both of whom were not officially given first names until the movies, and the former not until this year), that character's name is fanonical.  Figwit the Elf, so named by LotR fans who watched the Council of Elrond and said, "Frodo Is Great … Who Is That?" is unnamed in canon and may be one of Elrond's sons, or one of his attendants, but whom the producers started referring to as Figwit because of the fanon funny.

So if your fanon isn't canon, what does that mean?  Nothing, really.  We're fans.  We're used to creating our own realities within the canon realities.  Most slash is fanon.  Much  het is fanon.  All of fanfic is based on or creates new fanon.  We can argue about it, but since we're all stuck with the same canon (whether we want to acknowledge it or not) there's not much point in flaming someone for not playing along with your particular version of fanon.  Go play somewhere else, set up your own sandbox, and hang out with other fans who think Dumbledore and McGonagal were truly and deeply in love and that Tonks was just a rebound for Remus after Sirius died.  It's your fanon, enjoy it with your friends.  Don't demand that everyone else adhere to it, and don't try bullying them into changing their fanon to suit yours, and we'll all get along just fine.

Anyway, who knows?  Maybe the writers will change their minds again, and decide the fanon is cool enough to use.  (see Figwit …  Actually, keep looking at Figwit.  Figwit's pretty.)  Maybe they'll see the swell of fan support for this 'ship that's out of left field and throw in a mention.  Or maybe they'll get huffy about our interpreting the canon from the wrong perspective, kill off the characters in question in retribution for our not spending the proper amount of time worshipping the text and 'shipping the couples they intended us to 'ship.  It's hard to tell sometimes what's going to make producers cranky, what's going to flatter them, and what they're going to slip in just for giggles that screws up the canon for the REST OF TIME, and what the directors will have the actors do when they deliver the lines, and what the actors will do in the background while they're goofing off because they're bored or hungover or massively pregnant and hiding behind shrubbery.

Canon's canon.  Authorial intent is malleable.  Fanon is fun.  Enjoy them, work with them, accept that not everyone agrees with authors or fanon, and know that canon can be looked at with many different viewpoints.  Play nice.