A botnet is a collection of infected computers that are remotely controlled by a hacker.
Once a computer is infected with malicious software ( a bot), the hacker can control the computer remotely over the Internet. From then on, the computer is a zombie, doing the bidding of the hacker, although the user is completely unaware. Collectively, such computers are called a botnet.
The hacker can share or sell access to control the botnet, allowing others to use it for malicious purposes.
For example, a spammer can use a botnet to send out spam email. The majority of all spam is distributed this way. This allows the spammers to avoid detection and to get around any blacklisting applied to their own servers. It can also reduce their costs because the computer’s owner is paying for the Internet access.
Hackers can also use botnets to launch a distributed denial-of-service attack (DDoS). They arrange for thousands of computers to attempt to access the same website simultaneously, so that the web server is unable to handle all the requests reaching it. The website thus becomes inaccessible. (See, Denial-of-service attack, Spam, Backdoor Trojan)