Attackers use “botnets” or networks of “zombie” personal computers, which they have infected with a virus.  Denial of service comes in to play when the controlled computers are instructed to contact target websites on a repeated basis until such target websites are overwhelmed. 

More sophisticated e commerce websites protect against these attacks by using special software that detects and shuts down suspicious traffic.

Protect your computers!  Viruses often attack and there are no apparent signs that your computer has been contaminated.  Software is available to identify and flag dubious traffic flow.  Many companies offer a free scan, so make sure you update your antivirus software and run regular scans.  Also, potentially affected businesses should purchase Cyber Liability insurance to protect against this risk.

Source: http://www.msnbc.msn.com/id/31803381/ns/technology_and_science-security/