What Does Botnet Mean?
A botnet is a group of computers connected in a coordinated fashion for malicious purposes. Each computer in a botnet is called a bot. These bots form a network of compromised computers, which is controlled by a third party and used to transmit malware or spam, or to launch attacks.
Techopedia Explains Botnet
Originally, botnets were created as a tool with valid purposes in Internet relay chat (IRC) channels. Eventually, hackers exploited the vulnerabilities in IRC networks and developed bots to perform malicious activities such as password theft, keystroke logging, etc.