Botnet
A herd of hijacked devices taking orders. Spam, floods, miners, and stuffing logins all rent the same cattle.
A bot, in this context, is a compromised computer running someone else's jobs. A botnet is a collection of them, steered without the owner's knowledge: spam, malware delivery, denial of service, credential stuffing, crypto-mining.
You do not 'join' one on purpose. You get volunteered by malware. Patch, MFA, EDR, and taking infected boxes off the network shrink both your chance of contributing and your chance of being the target.
Fact source: ASD's ACSC glossary.