For about a fictional work too small to be called a novel - usually less than 10-20 thousand words and no chapters. Only use when identifying a work (use in conjunction with [story-identification] here) or when there is a tag ambiguity. If you are looking for a particular anthology of short stories, use the [anthology-book] tag.
There is no clear boundary between short story and novella, or novella and novel, but the most important difference is in length. Short stories tend to be less than 10,000 words, or sometimes up to 20,000.
The restriction on length means that short stories also have other attributes not shared by novellas and novels. The structure must be simpler, often centring around a single event and without much backstory. The focus is usually on a single character and story, without subplots.
Many short stories are sometimes gathered together in an anthology.