Flash fiction are stories less than a 1000 words and come in many varieties with sometimes very specific rules. Some other names are: short-short stories, sudden, postcard, minute, furious, fast, quick, and skinny fiction. Many have websites where you can share the torture of confining your writing by word count:
365 Tomorrows posts a new flash fiction everyday. You may submit your (science fiction based) stories for consideration.
Camille Renshaw provides a good overview of the craft in The Essentials of Micro-Fiction.
Here are a few types of flash fiction with specific limitations:
- pinhead stories (50 words or fewer)
- nanofiction (less than 50)
- 55 word (55 or fewer but must include a setting, character or characters, conflict, resolution, so it's not, for example, a slice of life piece.) You can read the 2008 winners of the 55-word contest run by New Times magazine for inspiration.
- 69er, 88er, 99er
- microfiction (under 100)
- drabble (100 exactly and its spinoffs: dribble - exactly 50 words, droubble - exactly 200 words) The Drabble Project has some examples from drabble's beginnings.
- ficleys (64-1024 words but you can continue someone else's story).