When you read nonfiction, you're reading about something that really happened — it's not a story somebody made up.
Prose is divided into the two big categories of fiction and nonfiction, and nonfiction includes all kinds of things — biographies, histories, memoirs, how-to books, self-help, books on business, even books on writing. Narrative nonfiction relates stories that really happened but in a way that draws you in just like fiction does; it tells a true story, but with lots of drama and all the interesting quirks of the characters.