A botnet is a collection of software agents that run autonomously and automatically. Generally, Botnet refers to a collection of infected computers or bots that have been taken over by hackers and used to perform malicious tasks or functions such as sending spam mails. The hackers are known as bot herders. World’s largest botnet , the most prolific source of spam e-mails that comprised an international network of virus-infected computers, called Rustock botnet” was shut down in 2011 in a series of coordinated raids by Microsoft and US federal authorities. Rustock botnet was capable of sending up to 25,000 spam messages per hour from an infected PC. For years it kept promoting unlicensed online pharmacies and cut-price anti-impotence pills.