Types
Botnet
A network of compromised machines under common command, rented out for spam, DDoS, fraud, mining and malware distribution.
A botnet is a fleet of infected hosts ("bots") controlled from common command-and-control infrastructure. Once enrolled, machines can be directed en masse to send spam, launch DDoS attacks, commit click/ad fraud, mine cryptocurrency, or distribute other malware.
Resilience by design
Mature botnets invest heavily in survival: peer-to-peer control, domain-generation algorithms, and fast-flux hosting to resist takedown. Families like Andromeda and Sality ran for years, and IoT botnets such as Mirai showed how quickly an army can be assembled from weak devices.
Defense
Block known C2, sinkhole DGA domains, patch and segment, and watch for the beaconing and scanning patterns that betray enrolled hosts.