It is a group of computers that have been taken over by criminal botherders. Instead of rounding up sheep, these crooks round up computers and use them for illegal purposes. The term stands for robot network also known as a zombie army.
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Cybercrooks gain control of unprotected computers by unleashing viruses, worms, or Trojan horses. The malicious software allows the crooks to use the computers to commit identity theft, put keystroke loggers on the compromised computers, launch attacks on computer systems and create havoc that can shut down a computer system. Some of the crooks use the botnets to send spam, one even went so far as to advertise his business using a botnet! One of the great concerns about the spam is that it often advertises prescription drugs that may be life threatening because they may be contaminated, ineffective, or have usage instructions that are wrong.
How does this software get into someone's computer?
It is so simple, you open an attachment, click on an ad, or respond to a phishing scam, these actions allow the crooks to gain access to and take over your computer. You aren't even aware your computer has been hijacked!
There are some things you can do to help protect your computer, such as using anti-virus software and anti-malware software that gets daily updates, disconnect your computer from the internet when you are away, and download free software only from sites you trust. Other steps can be found here.
Even when a botnet is disabled, the intruder must be removed from personal computers.
If you think your computer has been infected here is a website that may be able to assist you: Microsoft Virus and Security Solution Center