The web's most important database.

The Domain Name System is just a big distributed database that enables the automatic translation of a host name into an IP address. Every domain name registered has an entry in the root name servers giving the address of the name server that is responsible for keeping records about the domain. This primary name server will contain records for all the IP addresses used for Web servers, Mail servers, where email has to be sent, it can hold records for the physical location of machines, and records for all the devices associated with the domain.

How did your browser find Arrowhead Systems?

It asked your ISP's name server. The ISP name server asked a root server 'where is the name server responsible for theinternet.org.uk' and got the reply ns1.theinternet.org.uk & the IP address 212.89.81.222

The ISP server then asked ns1.theinternet.org.uk and was told the IP address for www.theinternet.org.uk. The ISP's server then remembers our IP address for a few hours in case any body else needs to know.

Unlike a phone book DNS allows you to find the reverse entry for an IP address, and lookup a machines name when you only know it's IP address, very important for anti-spam email security amongst other things.