Short answers, pulled from the story.
Canon refers to the body of works that take place within a specific fictional world and are widely considered authoritative. These stories usually come from the original author or developer who created the setting.
The Royal Book of Oz appeared in 1921, a year after original creator L. Frank Baum died. Ruth Plumly Thompson wrote the entire book under contract with publisher Reilly & Lee.
After the 25th of April 2014, Disney reclassified almost all pre-existing Expanded Universe content as non-canonical Legends. A new timeline emerged under the oversight of the Lucasfilm Story Group.
The Star Trek website defines canon strictly as events occurring within live-action episodes and movies. Tie-in novels, comic books, and video games are explicitly excluded from the main Star Trek canon despite occasionally influencing later television plots.
Treehouse of Horror episodes present fictional scenarios that do not affect the regular timeline of The Simpsons. These Halloween-themed specials allow characters to experience events impossible within standard continuity rules.
Fan fiction never holds official canonical status within any fictional universe. Certain ideas gain influence or wide acceptance among fan communities who label them as fanon.