A botnet is a collection of internet-connected computers whose security defenses have been breached and control ceded to a malicious party. Each such compromised device, known as a "bot", is created when a computer is penetrated by software from a malware distribution; otherwise known as malicious software. The controller of a botnet is able to direct the activities of these compromised computers through communication channels formed by standards-based network protocols such as IRC (Internet Relay Chat) and HTTP (Hypertext Transfer Protocol).
Read more about Botnet: Recruitment, Organization, Formation and Exploitation, Types of Attacks, Countermeasures, Historical List of Botnets