Flash fiction is any work of fiction that is typically 500 words or less. It is written in such a way that you get the maximum amount of impact in as few words as possible.

The "First they came for the Jews..." thing about the Nazis is a good example of flash fiction. It is under 100 words and it has a plot. The fewer words, the more concise, the better.

It may seem simple, but it's quite challenging. A few online zines publish flash fiction, such as Ideomancer (speculative), and Vestal Review (all genres). Typically paying .03 per word for 500 words or less and raising in respective categories until you get to 100 words or less, which typically pays .10 per word.

It's good when you're just looking for bite-size fiction tidbits. However, the really well-written ones are worth taking more time to examine, because a very skilled writer can conceal much more meaning in a few sentences than one might notice at first, or even second glance. That's what flash fiction is all about.