Right now I'm in the middle of a flash fiction contest running on an online writing group I belong to. Flash fiction is the term for a short-short story. Some people define it as anything less than 1,000 words, others say less than 750. Some publications want stories that are exactly 100 words long, or no longer than 500, or exactly 666 (muahaha!), and on and on and on. However a person defines flash, it's gotta be short.
Flash is the hardest length to write, in my opinion. A short story has much more leeway, a novella even more, and in a novel you can explore characters and setting and plot to your heart's content. Is your magnum opus running over 150,000 words and still showing no signs of ending any time soon? No problem! Make it a series!
But flash is short. It has to fit character and plot and setting into a page or two. I've written a lot of flash, but only published three stories at that length. A lot of times it ends up coming across as a vignette, or the beginning of a longer story, or I try to cram too much into too little of a space. It's like somebody handing you a lunchbox and telling you to pack everything you need in it for a month's vacation in Europe.
While it's difficult to do well, flash can also be fun. It's the perfect length to experiment with. A structure or premise that would get annoying if it were longer than 500 or 700 words can work as a short-short. And when flash is done well, it stays with you for a long time. Some stories that stand out for me include Todd Vandemarks's "Let Slip the Dogs," Desmond Warzel's "Wikihistory," and Annie Bellet's "Lists." Bruce Holland Rogers is quite prolific when it comes to writing short fiction, and he has an excellent series of posts on writing flash at Flash Fiction Online.