SpamBayes is a tool used to segregate unwanted mail (spam) from the mail
you want (ham). Before SpamBayes can be your spam filter of choice you
need to train it on representative samples of email you receive. After
it's been trained, you use SpamBayes to classify new mail according to
its spamminess and hamminess qualities.
from Will Maier (MAINTAINER), with some small tweaks by me