A novel is a long work of prose fiction—commonly treated as about 40,000 words or more—while a novella sits between a short story and a novel. Length cutoffs vary by publisher and award, but the Science Fiction and Fantasy Writers Association typically defines a novella as roughly 17,500 to 40,000 words.
Beyond word count, novellas usually focus on fewer conflicts and characters than novels and aim for a more concentrated narrative that can often be read in one sitting. Classic examples often labeled as novellas include Animal Farm, The Metamorphosis, and Of Mice and Men.
Source: Novella — Wikipedia