DDOS stands for Distributed Denial of Service. A DDOS attack is when thousands or tens of thousands (more in some cases) of computers maliciously try and connect to a specific computer or group of computers at the same time. The result is that the victim computer(s) became so swamped with the malicious traffic genuine users are unable to use or access those computers.

The computers used to attack are quite often home computers whose owners unknowingly have malicious software installed which is remotely controlled to take part in an attack. A good anti-virus product should be able to detect and remove the malicious software, while a good firewall product should also be able to prevent the malicious software from accessing the Internet to receive instructions and/or take part in an attack.