A short story is a story that's short, or bellow around 40,000 words. A novel is a work of fiction over 80,000 words. A novella is usually between 40,000 and 80,000 words.
Thanks Lord Lemex. one more question. When typing out your chapters, do you use a white background with black ink or do you use a dark background with say a white ink. This will be on a word processor. I am thinking of trying a black background, will let you all know what I think, will have to check the print out colour or change it back.
White background, black ink. I don't see why you would bother wasting so much ink to make your paper black, or white ink if you have black paper - but still, what's the point? And I'm not sure most publishers I know of would accept that anyway.
There are different definitions of story length in various places. Different publishers define things differently, and expectations vary from genre to genre. In general, though, you could probably say that a short story is less than about 10K words, a novella is between 10K and 50K, and novels are over 50K. That said, there are lots of genres where you'll have trouble selling a 50K word story as a novel. In what genre are you writing?