Botnets are networks of private computers infected with malicious software and controlled without the owners' knowledge. They are commonly used to launch distributed denial-of-service (DDoS) attacks and crack password files using brute force. An attacker first establishes command and control servers, then spreads bots to vulnerable devices using protocols like IRC and HTTP. Large botnets of thousands of devices allow password files to be cracked much faster than by a single computer. Common bot attacks include DDoS, spyware, click fraud, and password cracking. Users can help prevent infection by using antivirus software, firewalls, and keeping systems up to date.