On Writing- What makes a story?

Any story, whether a novel, novella, or novelette, requires characters, conflict, and resolution. We'll leave short stories for later.
The characters should be clearly pictured in your mind so that you can describe them clearly to readers. On occasion, I've resorted to images that I find on line in order to get a picture of a character, but I didn't have to do that with my first four books. It may be useful to keep notes on a character's appearance, personality and motivations. In my own case, I keep any notes on paper so that I don't have to open a new file if I have a question while I'm writing.
Conflict can come in two forms: External, where two or more of the characters are at odds over something, or internal, where a character resists doing something that they may very much want to do. Other characters may influence the conflicted one in one direction or another.
Resolution can be tricky. A Happily-Ever-After (HEA) ending may leave readers satisfied as long as it doesn't feel artificial or arbitrary to them. An unhappy ending may feel more 'real' to a writer, but not every reader is going to like it. Sometimes they work very well, though, and sometimes it's the only realistic way a story can end. There's also the ambiguous ending, where little or nothing is actually resolved, but at least one of the characters has changed in some way due to the conflict. These sorts of endings are good for promoting sequels, but if you overdo it and just churn out one yarn after another without ever resolving anything readers will grow tired of it all and start reading something else, probably by somebody else. If you absolutely have to go to an ambiguous ending, and don't plan on any sequels, it's best to leave at least the possibility of an HEA for a reader to console him or herself with.