It is extremely hard to define a novel. Perhaps impossible. Once an authority had offered a definition, some writer would be sure to come out with a novel that contradicted it. Henry James once said:
The only thing we should require of a novel is that it be interesting.
It is fairly easy to define a short story. So a novel would be a long story, a story that isn't a short story. Here is a good definition of the modern short story based largely on the dictum of Edgar Allan Poe, who is often described as the "father of the modern short story," or in some such words:
A short story is a dramatic narrative intended to be read in a single sitting and designed to produce a single effect.
A novel is not necessarily intended to be read in a single sitting or necessarily designed to produce a single effect, although some novels can be read in a single sitting. Poe meant by "single sitting" not much longer than an hour. He didn't believe that most readers could retain the same degree of interest and involvement for much more than an hour. In college we see that lectures are usually only about fifty minutes long. If the lecturer goes on much longer, we hear binders clicking and see all kinds of restlessness in the classroom. People can only absorb so much. They start thinking of other things. They have to go to the bathroom or smoke a cigarette or meet a friend--or something.
Short stories are intended to be read in a single sitting in order for the author to produce the intended single effect. We often get interrupted while reading a story, and then we will probably not experience the single effect, unless perhaps we start reading the story all over again. For example, we might start reading a short story in The New Yorker in a waiting room--and then the nurse comes out and says, "The doctor will see you now." Even if we swipe the copy of The New Yorker, we won't get that single effect if we were interrupted part way through.
The single effect (which might be called the single emotional effect, or "feeling") is the most important part of the definition of the modern short story--and it comes directly from the great Edgar Allan Poe. Some of his own short stories serve as good examples of the single effect he was talking about. These stories include "The Cask of Amontillado," "The Tell-Tale Heart," and "The Black Cat." A good way to write about a short story is to analyze the single effect it had on you. Shirley Jackson's famous story "The Lottery" is an excellent example of a short story with a memorable single effect.