Questions about stories with a fully developed plot, themes, and characters, but significantly shorter and less detailed than a novel.
A short story is a prose piece of fiction, generally short enough to read in a sitting easily. The short-story tag should be used on questions about short-stories in general, or about any specific short-stories. From this meta post:
For questions about short works, use the author tag with poetry or short-stories. New users will be able to tag their questions appropriately even if there've been no previous questions about precisely the same work, and we won't be overrun by thousands of overly specific tags. (Tags for specific, perhaps especially noteworthy, short works are optional but not encouraged.)