A bot is a compromised computer which is running software that allows a remote computer to control it.
The software is usually installed by someone breaking into the computer, but can also be installed by a user who thinks they're installing a game or other software which they have downloaded from a website or recieved via Instant Messenger
It is often difficult to detect a bot infected computer without looking at the network traffic it is generating.
There are many different kinds of bot software (and most bots can update themselves automatically), new versions are always being created, and the bot software itself can take steps to avoid being detected, so frequently the bot software cannot be detected when a computer is scanned for viruses and spyware.
