A botnet is a network of Internet connected computers that have been covertly usurped to forward transmissions to other computers on the Internet on behalf of a “master”. These transmission can be merely annoying such as spam or they can critically impact a target as when used to launch a Distributed Denial of Service attack.

Any such computer is referred to as a zombie - in effect, a computer "robot" or "bot" that serves the wishes of some master spam or virus originator. Most computers compromised in this way are home-based.

According to a report from Russian-based Kaspersky Labs, botnets -- not spam, viruses, or worms -- currently pose the biggest threat to the Internet. A report from Symantec came to a similar conclusion.