A botnet is a group of infected computers
A botnet consists of a large group of computers that are infected with a special virus. The botnet virus hides on the computer and makes every effort to go unnoticed. What it does is make itself available for the making of the virus. It can take over your computer and use it for example, a DDoS attack or sending spam messages. After the infection is the computer part of the botnet is is therefore called a zombie computer. Bots and botnets are a common form of internet crime and a powerful means of hackers. Botnet can be small and consist of a few hundred zombie computers but there are also botnets that consist of hundreds of thousands of zombies.
Symantec Guide to Scary Internet Stuff-botnets [YouTube] BRhauoXpNSs [/youtube]