Denial of service attacks are done by individuals flooding a web server with false and untraceable requests for information, overwhelming the system and ultimately crashing it. A distributed denial of service attack is where the attack is launched from several computers, usually without the computer owners being aware that their machines are being used in this way.
The 13 core servers are the "address book" of the internet's domain name system, which matches web addresses with computers' numerical codes and directs e-mail and other internet traffic to its destination. Internet traffic is normally managed by local servers, but if a domain is not found, the root servers are consulted.
It appears that Monday's attack lasted for about an hour. Although seven of the 13 servers failed to respond to 'legitimate' traffic, internet users worldwide were largely unaffected.
According to media reports, the servers worst affected were those operated by ICANN and the US Department of Defence Information Center.
The FBI's National Infrastructure Protection Center said that it was aware of the attack and that it is addressing the matter.