Bloom Filters by Jason Davies.
From the post:
Everyone is always raving about bloom filters. But what exactly are they, and what are they useful for?
Very straightforward explanation along with interactive demo. The applications section will immediately suggest how Bloom filters could be used when querying.
There are other complexities, see the Bloom Filter entry at Wikipedia. But as a first blush explanation, you will be hard pressed to find one as good as Jason’s.
I first saw this in a tweet by Allen Day.