I first came to know of the concept through email (Spammassassin, specifically), and now I’m interacting with it again due to my buddy Alex Hutton and the FAIR Risk Assessment system. Anyway, here’s what Bayes’ Theorem looks like when applied to email (from Wikipedia):.

Bayes’ theorem, in the context of spam, says that the probability that an email is spam, given that it has certain words in it, is equal to the probability of finding those certain words in spam email, times the probability that any email is spam, divided by the probability of finding those words in any email:

That’s an email version of this:

Very cool stuff. Here’s the main article on Wikipedia.