What are botnets?
A botnet is a connected network of computers – usually controlled by a command and control center – that communicate together in order to accomplish certain tasks. Although not always malicious, they are frequently used for illicit activities.
What they are used for
Where bots are used for illegal pursuits, the botnet is usually operated by a hacker, who then takes control of other computers within the network, referred to as zombies.
Computers that are part of a botnet are used to send spam, carry out denial-of-service attacks, and even transfer funds for criminal activities. The hacker can also sell the services of the botnet to send spam. This enables spammers to avoid detection (the emails are not coming from their servers), and cut down on costs, as the owner of the infected computer will be paying for the Internet service.
High-profile botnets that have been shut down include Grum (which was responsible for 26% of spam traffic between 2008 and 2012), GameoverZeus, ZeroAccess, and Kraken.
How to block botnets
Avira Free Antivirus protects you from botnets.