Definition: A botnet is a collection of Internet connected computers that have had software secretly installed that allows a remote controller to issue commands. These are typically sent encrypted. All the computers in the botnet (known as Zombies) can be used to send spam emails, or launch DDOS (Distributed Denial of service) attacks on websites or other servers.
This is done typically for financial gain. The infected computers are typically infected through a vulnerability in the system software. This can happen in some cases just through visiting a specially crafted webpage; this is known as a "drive by download". Once installed, the software then connects to a prearranged server and registers itself as ready to do nefarious stuff.
