description found at http://bogofilter.sourceforge.net/ :
Bogofilter is a mail filter that classifies mail as spam or ham (non-spam) by a statistical analysis of the message's header and content (body). The program is able to learn from the user's classifications and corrections.
The statistical technique is known as the Bayesian technique and its use for spam was described by Paul Graham in his article A Plan For Spam in August 2002. Gary Robinson, in his weblog Rants (September 2002), suggested some refinements for improved discrimination between spam and ham. Bogofilter's primary algorithm uses the f(w) parameter and the Fisher inverse chi-square technique that he describes. Paul Graham's new article Better Bayesian Filtering (January 2003) suggests some useful parsing improvements.
Bogofilter is run by an MDA script to classify an incoming message as spam or ham (using wordlists stored by BerkeleyDB?). Bogofilter provides processing for plain text and html. It supports multi-part mime message with decoding of base64, quoted-printable, and uuencoded text and ignores attachments, such as images.