Botnet is a technical term given to a collection or group of software agents or in other words robots that function autonomously.

The term is often and commonly used for a network of infected systems often called zombies. But the general idea is that of a network of computers that allow for distributed processing of any task.

The botnets function through a chain of command where the controller is called the master of the botnet or command and control center and the other systems that follow or execute the commands are called slaves (often called zombies).

Botnets may vary in size depending upon the complexity of the bots used. There have been cases where large botnets have been known to have hundreds of thousands of zombies to as small as a few hundred zombies.

The infection method is similar to that of virus or Trojan infection. The users who are not too security savvy might receive an attachment from some unknown sender and without due consideration if the attachment is opened, it might contain the bot agent which would get installed on the system and make the system a zombie.

Most of the cases of infection occur when the system is already affected with some kind of a Trojan or rootkit and that particular malicious code would download the bot agents and make the system part of a botnet.