Although I usually write poems these days, I started off exclusively producing short stories. It took a year of writing verse before I’d call myself a poet. However, I found myself going back to stories after a long time away.
There is no universally-accepted definition of a short story: some focus on the word count, while others consider whether the story could be read in a single sitting.
In any case, there are some features that distinguish this form from longer prose:
Even a slow or meandering short will make its point more quickly than a longer story. A 2000-word story might spend 500 words introducing the concept, the next 1200 might explore how the status quo is upset, while the remaining words resolve the story and often spring a twist upon the reader.
In a novel, the first chapter alone could be 2000 words.
Every word plays a part
While there is scope for description in a short story, there probably won’t be room to include detail that isn’t directly relevant to the plot. For example, the reader probably doesn’t need to know the main character wears a yellow scarf and a green clip unless those items are later found at a murder scene.
Characters and locations are limited
In a short, it’s rare to find more than five characters or a number of different locations, otherwise the story can feel as though it’s jumping around too much. I novel, on the other hand, can change location every chapter if the plot demands it.
If you’re writing and you find you can’t keep within these constraints, you might have a novella on your hands or even a novel. Let it develop any way it comes out.
Generally, the more words you write, the more description, plot and characters can be included without overworking the narrative.
Incidentally, it’s easier for a filmmaker to adapt a short to the screen than a novel because less action needs to be left out. It’s a Wonderful Life, Total Recall and Brokeback Mountain are all based on short stories.