Botnets are collections of internet-connected computers that are controlled by cybercriminals without the owners' knowledge. The document discusses how botnets work through command-and-control servers, the threats they pose such as distributed denial-of-service attacks and spam, and methods for detecting and preventing botnet infections and activity. It also analyzes the findings of a study on botnet technologies, including their propagation, exploits, evasion techniques, and implications for security research.