What is a Botnet?

A bot is an automated software program that performs specific tasks on a network with some degree of autonomy. Bots can perform many beneficial and even vital functions. For example, the web crawling software programs used by popular search engines to index web pages but unfortunately, bots can also be developed for malicious purposes, such as assembling networks of compromised computers—botnets—that are controlled remotely by one or more individuals, called bot-herders.

Computers in a botnet, called nodes or zombies, are often ordinary computers sitting on desktops in homes and offices around the world. Typically, computers become nodes in a botnet when attackers illicitly install malware that secretly connects the computers to the botnet and they perform tasks such as sending spam, hosting or distributing malware or other illegal files, or attacking other computers. Attackers usually install bots by exploiting vulnerabilities in software or by using social engineering tactics to trick users into installing the malware.

Users are often unaware that their computers are being used for malicious purposes. Victims who detect suspicious activity on their computers are likely to take steps to find and fix the problem, perhaps by running an on-demand malware scan or by updating the signature files for their existing real-time malware protection.

Read more here – http://www.microsoft.com/security/sir/story/default.aspx#!botnetsection