An essay is a particular type of writing. It's typically a short block of writing devoted to a single subject or group of subjects, used as a rhetorical tool. That is, a five-page paper you wrote for History class would be called an essay, but Lord of the Rings is not. Prose is simply the word used to describe a STYLE of writing. The opposite of prose is poetry, and the two do not usually collide. Basically, prose is just ordinary writing with proper grammar and no rhythmic structure (I'm writing prose right now), while poetry is the exact opposite. A novel is typically a large book used to tell a single story or several large stories. Books with many short stories in them are not novels, and stories in smaller books are novellas. However, it's not necessarily fiction. You could, for instance, write a book about the life of Winston Churchhill and it would still be considered a novel as long as it's presented as a story instead of a rigid history book. Fiction is any story that does not adhere to observable, empirical truths. This means that fiction encompasses just about everything. If what you're reading isn't a textbook, a newspaper article, or a how-to manual, it's probably fiction. In fact, if the things mentioned in those aforementioned items aren't true, they're ALSO fiction. Basically, fiction is anything that makes stuff up for whatever reason. A novelist is a person who writes novels. If a man has large books that have over 300 pages each, then he's probably a novelist. An essayist is a person who writes essays. If a woman has only ever written persuasive arguments on specific topics (typically not over fifty pages, but there are exceptions), then she's probably an essayist. Some people can be both, however. Many popular novelists, like Mark Twain, also wrote a lot of essays, so you could call them by both titles. Philosophers also typically write both essays and novels, as do some scientists, economists, and social critics. That is, just because a person is a novelist doesn't mean they can't also be an essayist, and vice versa. I hope this answered your question. If you need me to simplify it further, I can try.

A "novel" is an imagined story (i.e., one that isn't true) and a "novelist" is someone who writes novels. "Fiction" is an imagined story style, but it can be a novel, short story, play, movie script, etc. An "essay" is a non-fiction (i.e., true) story based on either facts or opinion, but not completely imaginary like fiction is. Essays are written by "essayists." "Prose" is any kind of writing that isn't poetry: novels, essays, stories, articles, etc.