As I mentioned in my previous article, each fandom has a canon. It depends on the fandom itself what is considered canon. In sports, their canon might be the rules and regulations that the sport runs by. In a fandom such as Star Wars some fans may consider only the films canon, others might include the extensive extended universe tie in novels. Game of Thrones fans might consider their canon either the show or the books, or attempt to meld the two.
There are other choices other then strict canon, such as fanon, semi-canon, Alternate Universe, and of course non-canon.
Fanon is basically fandom created canon. These are character traits, or information that fans themselves have created and have chosen to believe as fact until the canon being developed proves them wrong.
An example of this was back before the last two books of Harry Potter came out, no one knew what gender the character Blaise Zabini was. Much of early fic therefore had the character be a female character. This would be considered fanon. Of course it became non-canon when JK Rowling finally mentioned that Blaise was in fact a male character. Blaise was also often (even when he was outed as a male character) described as a very good looking white Italian until canon once again told us he was in fact a good looking black man.
Fanon and Head canons often run in the same circles. Headcanons are basically fanon, just on a more personal level. These, like fanon, are elements of personal thoughts to fill in missing parts of the story till the canon gives us the answers.
Headcanons are often confused with Alternate realities. Headcanons are elements of the fandom that are neither proved nor disproved by canon. If they become part of canon, they become canon, and if they are proved incorrect about Canon (such as “I think character X has a large family’ and then you find out that character x is an only child or only has one sibling) it becomes non-canon. I have a personal headcanon that Chris LaSalle’s older sister makes ceramic ornaments and he has a personalized one made for each member of his adopted family. There is nothing within the canon of NCIS: New Orleans to say she doesn’t. Headcanons develop characters, especially minor ones, in ways the original material often can’t. There isn’t enough time or space to fit all the little quirks about a character, about their backstory.
People often enjoy creating headcanons about characters to fit unique parts of the fandom that don’t see much representation. Like saying a character is autistic, or bisexual. However, I see many of these go in the face of evidence from the original material which makes them alternative universe.
Alternate Universes are fun. I like to play in them myself. Usually one element of the story is changed. There are different kinds of Alternate universes. There are the kinds where you change the setting or Time period. Game of Thrones fandom likes to make modern adaptions of the stories. These are alternate universes, with varying dependence on Canon. The infamous Coffee House AU, where everyone seems to work in a coffee house/bar/restaurant is one such alternate universe.
Some people decide to write semi-canon stories where one element is changed. Like what if Anakin Skywalker had been brought back to the light. Had been able to raise Luke himself rather then the Lars family. Or what if Harry had accepted Draco’s friendship at the start of the books. These can be considered Alternate universe as well.
Semi-canon in general means that for the most part the story uses canon elements, such as setting. time period, character traits, etc. But that some things are changed. These can be stories that were started over a hiatus to theorize what will happen. Or it answers the question ‘what if that hadn’t happened and this happened instead.’
Non-canon of course is anything that is not canon. These include personal preferences about a character’s sexuality, mental space, or traits that doesn’t seem to fit the evidence in the original material, or giving them a completely new story and just using the characters or other elements of the story that are just against whats there. Sometimes AUs can stray too far from the canon established to be considered canon any longer.
An example of this would be if the Mirror Universe were not actually a part of Star Trek. If it was just a story someone wrote as a fanfic, and we saw Kirk being so vicious, or T’Pol and Trip being moderately abusive towards one another as they are in those episodes, that would be consider non-canon.
None of these are bad, but you have to be clear about what you are writing. People tend to look for specific things. They might enjoy a story where a character is gender bent. Or they might just want an episode-related tag about something that needed fleshed out.
Fanfic is about having fun with the material, being able to play with it and mold it to fit what you see, to make commentary on what you take in. Its something to enjoy, both as a reader and a writer.