The Domain Name System is how the Internet links together the thousands of Networks that it is comprised of. The DNS is utilised whenever you send an Email or access a particular Web Page . Each computer on the Internet has a one of more Domain Names such as "fredbloggs.co.uk". The .co indicates a commercial organisation & the .uk indicates that the computer is in the United Kingdom.
Standard conventions used in Domain Names include:-
ac - Educational institution
co - Commercial organisation
com - Commercial organisation
edu - Educational institution
gov - Non military government organisations
int - International Organisations
mil - Military government organisations
net - Networks
org - non profit organisation
You will also see these codes in URL's such as
"homepages.enterprise.net/jenko/Glossary/G.htm".

These DNS converts the Domain Names to a unique number known as an IP address (the IP stands for Internet Protocol). You will often see the IP address displayed by your Web Browser when you are connecting to a particular computer.