The words "story" vs. "novel" might seem interchangeable, but there are real differences between them. While novels do tell stories, not all stories are novels.
Let's explore what each one really is and how they relate to each other.
A story is an attempt to communicate something to an audience using representations of life in action.
The same story can be told in any number of different formats, from campfire tales to magazine stories to books to TV shows and movies. Even photographs and advertisements can tell stories.
The story is the thing in your mind that you want to share, not the format you use to share it. The novel or the campfire tale or any other format is just a delivery method.
Told around a campfire, the same story would have a very different form vs. when told as a novel, but it remains essentially the same story. Consider "The Headless Horseman", for an example.
Whatever form you use, your audience takes in all the facets of your delivery method and uses them to recreate your story in their own minds.
The campfire audience takes in the words the storyteller uses, the way her voice and body language modulate the delivery, the way the campfire environment helps reinforce the mood, and so on.
Likewise, by reading your novel, readers recreate your story in their own minds out of the synergy of all the words you strung together and all the techniques you used to do it, such as description, flashbacks, foreshadowing, theme and stakes and conflict, POV and narrative voice, and so on.
Regardless of format, you use every tool at your disposal to immerse your audience in your story, getting them to experience it just the way you see it in your own mind.
Writers tell stories by writing them down.
For us, a story is a written account of what happens to one or more characters. It's a unit of adventure that carries our meanings to our readers via the words we put on the page.
Our stories can be fictional or true life. They can be calm or flashy or gentle or violent, or any other description we can imagine. They can be any length from very short flash fiction to continuing series that stretch on for many volumes.
A novel is one of the many formats you can use to tell a story. It has a range of attributes that distinguish it from all the other possible story formats.
It's written instead of filmed or painted or spoken, etc., -- but so are several other formats, such as a short story or a script.
So, what factors make a written story into a novel rather than some other written format?
Length is the most obvious factor. Novels are long, by definition.
Novels can range from the relatively short (less than 100 pages) to the very long (easily 1000 pages or more).
The short end
As the novel gets shorter, you gradually loose room for many of the internal factors that characterize novels, such as indulging in rich description, exploring subplots, and developing deep character history and motivations.
At some point, the text gets so short that you have to acknowledge it's not a novel or novella, but a short story, instead.
(A short novel (or a long short story) is often called a novella. It's an in-between term referring to the gray area between what's clearly long enough to be a novel, and what's clearly short enough to be a short story.)
The long end
On the long end, there isn't any particular limit to the length of a novel -- but at some point, you will run out of practical ability to print the novel as a single volume. A very long novel often gets split into multiple books for convenience and marketing reasons -- it becomes one of several kinds of series.
At that point the word "novel" begins to apply to each separate volume rather than to the series as a whole, even when the entire series comprises one long story.
Aside from length, there are a number of less obvious factors that define novels as separate from other story formats. Here are some of them.
Written as books
Novels are created in written form and presented as books. There are other words for other formats, such as play, film, TV drama, podcast, etc.
Written in prose
Novels are written in prose rather than verse. And novels normally use regular speech patterns rather than poetic techniques such as rhyme and meter (though there are exceptions).
Fictional
Stories can be real world true, as in news stories, histories, or biographies. Or they can be fictional like Cinderella or Star Wars. But novels are, by definition, fiction. A biography of J. K. Rowling could be printed as a book, but it wouldn't be a novel.
Story-focused
Many non-fiction books are focused on educating the reader, passing on facts rather than telling stories. Any stories they include are incidental to the facts, meant to provide context or relevance in support of learning those facts.
Novels always tell stories. Any facts they pass on are presented incidentally to and as support for the flow of the story, not as being important in and of themselves.
Entertainment over education
No matter how urgently the writer wants to convey her underlying message, or how serious and important that message may be, novels are focused on entertaining the reader. They are about creating enjoyment, and providing diversion ahead of trying to teach. Whatever author messages they may contain work best when passed on subtly, between the lines.
If the novel format isn't direct enough to convey your message the way you want, there is nothing wrong with delivering it through some other format instead. You could write a manifesto, or a give a speech, or teach a course. Each of those formats can still deliver stories, though they are not novels.
Storytelling is the act of relating the events of a story.
Novel writing is storytelling via a great many written words.
The length of a novel gives you leeway to take advantage of the fine details and nuanced complexities that shorter forms are forced to ignore, and that some non-written forms simply can't express.
Stories can be told in any number of formats. You choose the format you use for each story to suit your artistic and practical objectives.
From there, it's the format that gives you the power to reach your audiences, to share your stories and communicate your message.
The novel's length and flexibility as an artistic format make it a powerful and versatile choice for all but the simplest and most direct stories you might want to tell.