A botnet is a network of computers or devices that have been infected with malicious software, allowing a hacker to control them remotely. These compromised devices, often referred to as bots or zombies, can be used to perform various tasks without the owner's knowledge, such as sending spam emails or launching attacks on websites. Botnets can vary in size, from a few infected devices to millions. They are commonly used for Distributed Denial of Service (DDoS) attacks, where multiple bots overwhelm a target server, causing it to crash. This makes botnets a significant threat in the realm of cybersecurity.