In simple terms, a botnet is a collection of machines that have been compromised by software installed by the attacker so that they now respond to commands sent by the attacker.
Computers typically get infected by botnet malware by a virus or worm. Individuals get these viruses and worms by opening up an infected attachment or by visiting an infected website. It has been claimed that most modern anti viral programs like AVG, Norton Antis Virus or Kaspersky Anti Virus, will keep a computer safe from attackers, however recent events appear to suggest otherwise.
Botnets are of increasing significance to the public in light of the fact that criminals use botnets to send out spam email messages, spread viruses, attack computers and servers and commit other kinds of crime and fraud. It is useful for the public to be aware that a computer that is part of a botnet may slow down and suffer from decreased performance.