A botnet describes a group of computers that has been connected via malware infections in such a way that a criminal can collectively control them from afar. Botnets are used to carry out various malicious tasks such as sending spam, launching phishing attacks, and distributing further malware. The controlling criminal is often referred to as the ‘botmaster’.

A computer that becomes part of a botnet is said to be a ‘zombie’ or ‘robot’ because the computer is under the control of criminals. Botnets are sometimes called ‘zombie armies’.  Botnets can consist of hundreds of thousands – even millions – of infected computers.

By using such an ‘army’ of infected computers, online criminals can distribute vast numbers of spam, scam and malware messages. And, these messages will trace back to the infected computers rather than to the criminals responsible for the attacks.

Typically, a computer becomes part of a botnet after the computer’s user is tricked into installing an initial malware component by clicking a link that opens a compromised website or opening an attached file.