The term "novel" derives from the Italian "novella", a word that was originally used to describe anything small and new. From there, the word evolved to meaning a short prose story, until the term came to represent the novel as we know it today. In this sense, we might say that short stories are the predecessors of today's novel, and even though this indicates that they share strong similarities, they also have important differences.
Similarities: Narrative Prose
Both the novel and the short story fall in the same category of prose writing -- narrative writing. With narrative being anything that is narrated, a story, it becomes obvious that both the novel and the short story share the same main narrative elements, such as plot, characters, theme and conflict. These two types of prose also share the same established narrative structure: the three phases of beginning, middle and end.
Since the 18th century, the novel has grown to be the most popular literary form, and in the same way as the short story, it has represented the social worries and fears of the majority of people, without being restricted to a certain class or people of a specific background. Both the novel and the short story always have a strong central theme, sometimes clearly stated and sometimes just implied, which touches a broad audience and gets a moral message across. Without a central theme, both these literary forms lose their purpose.
Conflict is crucial to the plot of both the short story and the novel. Without conflict there is no force connecting one event to the next, no reason why everything happens, which means no plot, thus no story. Both the novel and the short story need conflict in order to create obstacles, to raise action and lead their characters and readers to a climax, the turning point of the story which will later bring the resolution.
The novel's length differs greatly from that of the short story. A short story should be able to narrate a story in less than 7,000 words, when a novel always exceeds 50,000 words and sometimes even reaches 100,000 words. So while a short story can be read in one go, a novel can only be read in stages.
Differences: Plot and Characters
The short story features a far less complex plot than the novel, which also requires fewer characters. Usually, a short story will deal with one incident at a specific time and focus more on creating a certain mood and atmosphere. On the other hand, the novel is more complex, dealing with a chain of closely connected events, involving more characters and more settings.
Differences: Exposition and Resolution
In longer forms of narrative prose, like the novel, stories start with an exposition, an introduction to the plot. Things after this get more complicated as the characters deepen and the plot thickens, until the conflict reaches its climax, and there is no other way to go but towards a resolution, where problems are solved and questions are answered. In the short story, however, the dramatic structure is different. Most short stories lack exposition and begin the narrative from the middle of the story, cutting straight to the chase. Very often a short story will have a sudden beginning and an abrupt ending instead of a meaningful resolution, focusing more on describing one event and creating a certain emotion, rather than answering questions and solving problems.