What is a botnet attack?

I'm asking this because Norton had sent me an email asking me to buy their new antibot real time protection system. And if you already have anti-virus and spyware protection, is this really necessary?

What is a botnet attack?antivirus software

A botnet (also known as a zombie army) is a number of Internet computers that, although their owners are unaware of it, have been set up to forward transmissions (including spam or viruses) to other computers on the Internet. 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 Kapersky Labs, botnets -- not spam, viruses, or worms -- currently pose the biggest threat to the Internet. A report from Symantec came to a similar conclusion.

http://searchsecurity.techtarget.com/sDe...

Botnet. From Wikipedia, the free encyclopedia.

Botnet is a jargon term for a collection of software robots, or bots, which run autonomously. This can also refer to the network of computers using distributed computing software.

While the term "botnet" can be used to refer to any group of bots, such as IRC bots, the word is generally used to refer to a collection of compromised computers (called zombie computers) running programs, usually referred to as worms, Trojan horses, or backdoors, under a common command and control infrastructure. A botnet's originator (aka "bot herder") can control the group remotely, usually through a means such as IRC, and usually for nefarious purposes. Individual programs manifest as IRC "bots". Often the command and control takes place via an IRC server or a specific channel on a public IRC network. A bot typically runs hidden, and complies with the RFC 1459 (IRC) standard. Generally, the perpetrator of the botnet has compromised a series of systems using various tools (exploits, buffer overflows, as well as others; see also RPC). Newer bots can automatically scan their environment and propagate themselves using vulnerabilities and weak passwords. Generally, the more vulnerabilities a bot can scan and propagate through, the more valuable it becomes to a botnet controller community.