Bloom filters

Perl.com has a good Bloom
filters, something I learned just this past week. In a nutshell,
Bloom filters are useful in dealing with gawd-awfully-large databases.
A Bloom filter will quickly tell you, accurately, if what you're looking
for is not in the database or, less accurately, if what you're
looking for might be in the database. Shorter version: it's a
way to avoid having to search massive databases for every query that a
user throws at a program.