Length
The most obvious difference between a short story and a novel is length, typically measured in the number of words contained within. A short story, as the moniker suggests, usually consists of only a few thousand words, while a novel can contain tens of thousands. The exact number differs depending on the specifications of the publisher, but a short story is usually somewhere between 2500 and 5000 words, while a novel can be upwards of 50,000.
Pace
Due to their relatively small word counts, short stories can't waste time with detailed descriptions or unnecessary details. The action has to start immediately, usually at the very beginning of the piece, and continue through to the resolution with no downtime. A novel, on the other hand, has time to gradually build up to the conflict and can delve deeper into the interactions and motivations of the characters, adding side details and background information throughout. In racing terms, a short story is a sprint, while a novel is a marathon.
Characters
Because of the word count constraints, short stories typically have only a few characters, with very little in-depth descriptions of their physical appearance or inner thoughts. The action has to happen quickly and carry through to the end, and a quick story is difficult with a dozen characters to keep track of. Novels are worlds of their own, though, with larger casts of main, side and recurring characters.
Plot
In terms of plot, a short story is like a half-hour television show, while a novel is more like a full-length movie. Short stories usually consist of only a few scenes: an introduction, conflict and resolution in relatively rapid succession. A novel has a much deeper plot, usually with a subplot or two intertwined. Novels incorporate richer character development than short stories can offer, furthering the plot with flashbacks, foreshadowing and inner reflection.