I absolutely love my stippling brush for applying my liquid foundation and I keep a separate stippling brush to apply my setting powder, too. The brush gives a nice, even distribution of products and leaves a nice air-brushed effect when finished. You can find a good one at: http://www.sephora.com/browse/product.jhtml;jsessi
I hope this helps you.
It's used generally to stipple on makeup. Essentially it mimics the effect of airbrushing, because as you "pat" or "press" makeup on with the brush all the bristles dot on a little makeup rather than streaking or wiping makeup on, similiar to makeup being sprayed on (in airbrushing). Usually it is used for liquid foundation or other liquid or cream makeup (I love them for cream blush).
I know some people "swirl" when applying makeup with these brushes, but they are really designed for stippling.(definition: to dot, fleck or make many small touches, usually in painting or engraving). But, hey, to each their own, you can use it for however you find works well! I certainly have used products and tools other than how they were originally intended because it worked.