A botnet is created when more than one bot is carrying out the malicious functions of a bot.
Botnets begin when bots are installed onto devices, generally undiscovered, using the infected device's computing power and connectivity to carry out a series of commands. As the number of devices recruited increases this enhances the activities for which the bot was deployed. These include credential-stuffing, distributed denial of service (DDoS) attacks, click fraud, rewards scams, digital currency mining, and auction sniping.
Once discovered, bots are removed by uninstalling them. This is sometimes a difficult undertaking as some bots are sticky and may turn one's device into a "Bitcoin mining zombie" that defensively keeps working while limiting the user's control over other functions.
"My computer was inaccessible so I took it in for service. They told me a bot was installed and that my laptop is part of a botnet that's doing click fraud. They uninstalled it and suggested I be wary of clicking email attachments that might have malware that installs a bot."