"Fiction" includes everything written that is not factual. If it is a story that the author has made up using his or her imagination, it's fiction.
A novel is one kind of fiction. A novel is, specifically, a long piece of fiction. A novel is usually an entire book, more than 50,000 words long, telling a long story.
Some other kinds of fiction are: short stories, novellas (longer than a short story, shorter than a novel), plays, and graphic novels.