A botnet is a piece of malware that is based on the client-server model. The victim machine that is infected with the malware is called a bot. The hacker controls the bot by using a C&C server. This is also called a bot herder. A C&C server can issue commands to the bots. If a large number of computers are infected with bots, they can be used to direct a lot of traffic toward any server. If the server is not secure enough and is incapable of handling huge traffic, it can shut down. This is usually called a denial of service (DOS) attack. A bot can use internet protocols or custom protocols to communicate with its C&C server.

ZeroAccess and GameOver are famous botnets of the recent past.