Across my website, I often use "fandom terms" that may be unfamiliar to you. This page serves as a glossary of sorts, but please keep in mind that it only covers what is relevant to me and my content. As such, it may contain gaps or inaccuracies in relation to other people's fanworks.
Canon: Refers to something that actually happened within a piece of non-fanwork media.
For example: It is canonical that Darth Vader is Luke's father.
Headcanon: Refers to something that an individual fanwork maker personally believes is true, but is not confirmed in canon, or even has little to no implication in canon.
For example: I, Kelsey, headcanon that Shulk from Xenoblade Chronicles 1 is on the Autism spectrum (or whatever the Xenoblade world's equivalent of that is).
Fanon: Refers to a headcanon that is popular enough to be widely known within a fandom, but is not confirmed in canon.
For example: It is fanon that Miss Frizzle from "The Magic School Bus" is a lesbian. (Her voice actress, Lily Tomlin, is married to a woman, by the way)
Ship: A romantic pairing of two or more characters together. Can be used as a noun or as a verb. Often signified with "/", but I personally use "x".
For example: I, Kelsey, enjoy shipping the Stardew Valley ship Maru x Penny.
OC: Acronym for "Original Character". Refers to a character made by the author of a fanwork, rather than a character from a pre-existing piece of media.
For example: Back in third grade, I had a Beyblade OC named Flower. She worked for Nintendo, constantly vandalized Wikipedia, and could turn video game consoles into chocolate.
OoC: Not to be confused with "OC". Acronym for "Out of Character". Refers to a character behaving differently than they would in canon. Generally frowned upon.
For example: If Indiana Jones willingly kept a snake as a pet, that would be extremely OoC.
AU: Acronym for "Alternate Universe". Refers to divergence from what happened in canon. Can range from small-scale (a character who died in canon still being alive in a fanwork), to larger-scale (a fanwork where entire canonical events are changed, or ignored entirely).
For example: This story takes place in an AU where aliens made first contact with the Earth in the 19th century.