What is bot net ?

A botnet is a distributed network of (infected) computers which can be manipulated to execute DDoS attacks and other malicious activities. These are people's computers which have a virus or trojan allowing back-door access wherein their computer can send/receive packets as part of a network without them knowing it.

They are harmful because they can be used to bring down large networks or send spam email without their owners' knowledge. These networks are often rented out to third parties to perpetrate these tasks.

Wikipedia has a great article if you're needing more specifics on what specifically a botnet is, and how it's distributed (referenced below).

http://en.wikipedia.org/wiki/Botnet