This is a recent discovery for me.
Micro fiction is a really short story, much shorter than flash fiction, and typically under 200 words. I've seen lengths vary - Alienskin Magazine look for stories of exactly 150 words (excluding title) while Necrotic Tissue request 100 words exactly including story title.
It's more difficult than it sounds to compress an interesting story or idea into such a tight word limit, and yet somehow leave the impression of a much bigger tale. Even more of a challenge when you have to meet an exact word count (you might have to add words as much as edit them!). It's fun though, takes less time to polish than a longer story, and there seem to be quite a few markets out there.
So have a go!