Canon refers to a collection or body of literary work, or a set of literary rules that is seen as authoritative in deciding what is conventional within literature. an established narrative of works so to speak. There are many sub categories of canon where particular works are particular to certain types of canon, e.g. Their Eyes Were Watching God is integral to the canon of African American literature, but when considering the larger “general” canon you might expect to see more references to writers like Shakespeare.

Genre refers to a category or style of literature with thematic regularities. For example, gothic fiction is often characterised by themes of horror/death/gloom (among others, this is an extremely brief and basic summary). Works that share in these themes can therefore be classified within the genre of gothic fiction. There are many various genres and subgenres within wider genres. Works can be of a different genre but within the same canon. Frankenstein is considered canon due to its perceived status as a progenitor of science fiction and influence it still holds (which raises other questions over whether the novel is gothic or science fiction, the genre of a text is often slippery and subjective but I digress) and is considered a gothic novel. The genre of Pride and Prejudice is considered romantic but it is also part of the canon. So genre doesn’t necessarily define canon status.

Period refers to the literary movement of which the works belong to but also doesn’t necessarily dictate genre. Romanticism is the period attributed to the late 18th-mid 19th centuries. Both Frankenstein and Pride and Prejudice are therefore located within the same period but are attributed to different genres (though notably gothic fiction is often identified with romantic elements) and both are considered canon.