A large number of computers that have been infected, and effectively hijacked, can be grouped together to achieve a common purpose.
This group of hijacked computers is commonly referred to as a ‘botnet’.

what is a botnet?

Cyber criminals, who are often part of global syndicates, can remotely control all of the machines in a botnet whilst retaining almost complete anonymity.
Botnets are often utilised for a variety of purposes, all without the user’s knowledge.
These nefarious tasks include -
§ sending huge amounts of spam emails
§ launching denial of service attacks against targeted websites
§ spreading viruses
§ stealing personal information for identity theft attacks
Additionally, because botnets are controlled remotely, they can be used to install software, such as keyloggers, which can then be used for monitoring keystrokes on a computer keyboard.
This will then typically yield passwords and information about various accounts, such as bank accounts and social networking profiles, allowing the controller(s) of the botnet to commit fraud or propagate the hijacking through other sites.
At this moment in time there is no easy method of disabling botnets as those behind them are extremely clever at hiding their work and re-routing information in order to evade detection.
There are a large number of botnets in existence across the globe, the most notable of which is probably Waledec which is linked to the notorious Conficker worm, and may have several millions of computers in its network.