Flash Fiction, as you probably know by now, is simply short fiction, but focused and direct, even shorter than short, typically less than 2000 words. Basically anything longer than Hemingway's famous "for sale, baby shoes, never worn" is probably flash fiction, up until maybe 1500 words, at which point it becomes a regular 'ol short story, which has, I was once told, an upper limit of James Joyce's "The Dead". That seems as good a place as any.
Flash fiction is just like regular fiction- it needs to have a unity of effect, as Mr. Poe taught us, and have a beginning, a middle, and an end. It's just shorter.
This Blog has practiced Flash Fiction in this space from time to time, with varying degrees of success. This Blog is most proud of, at present, the fact that It has not murdered Its teenage son, as well as the works of flash fiction it has propagated in the marvelous 2014 series, now available in your favorite electronic or dead tree format.