Spam Hound uses information provided by millions of other email users to better detect spam.

This is a technique by which the people receiving spam can cooperate to defend themselves against spammers.
If a very smart spammer manages to evade other filtering techniques by crafting a very sophisticated spam.
He can be thwarted by having the first person that receives the spam identify it as spam.
Spam Hound will subsequently filter it out for all other email users using Spam Hound.
Categorizing a message as spam takes a single click of the mouse.

To preserve privacy Email is shared in the form of "Checksums" which are sort of fingerprint of the email.

Spam Hound uses two such cooperative filtering systems: the Distributed Checksum Clearinghouse (DCC) as well as Apocgraphy's proprietary database.
Currently more than 150 million email pass through the DCC on week days.

Cooperative filtering is extremely effective though it suffers from the problem that people sometimes disagree on what is spam and what is not.
Spam filters that rely solely on cooperative filtering run the risk of blocking email that you do not consider spam.
Spam Hound on the other hand uses cooperative filtering as just one of a number of filtering methods and automatically learns your email preferences.