The most obvious answer to this question is length. It’s not that helpful of an answer though. But it is an answer that informs all the helpful ones.
Short stories and novels both lead up to something.
In novels, there is generally a conclusion.
In short stories, it can be a moment, a revelation, a decision. It can be a conclusion. But it doesn’t have to be .
People get annoyed when a book doesn’t have a conclusion. “I’ve invested hours in this, read over 80k words, and you’re just going to leave me hanging?!”
Short stories only have that problem once they approach a certain length. The shorter the story, the less need there is for everything to be tied up neatly with a bow. People are waiting for what you’re leading up too, sure, but they don’t need to know what happened after that moment.
The shorter the story is, the more versatile the word “story” gets. A list can be a story if it implies enough. A recipe with annotations. A moment of revelation. The description of an event.
All a short story has to be is interesting. The longer a piece is, the harder it is to keep a readers interest. It can no longer just be surprising, or interesting. Conflict come into it. Conflict without an antagonist can only get so far before it becomes melodrama. Conflict with an antagonist involves readers in the characters, makes them care. Once people care, they want to know what happens to characters. If you make them care for a long period of time, about multiple characters, a resolution off that conflict, of those characters problems will be needed. It is no longer enough just to be interesting.
The shorter a piece, the less conflict needed. That doesn’t mean a lack of conflict, it just means you don’t need as much of it. You also don’t need an antagonist to propel all that conflict. As the story becomes longer, it can no longer be just interesting. In order to get the reader to invest in what you’re writing, you need to do more than imply conflict. “Baby shoes for sale, never worn” does not have conflict in it. But it sure implies a lot.
Picture taken from Flickr, under creative commons, attributed to Dennis Wilkinson