A Subdomain is used by web sites to increase the ranking of sections of their web site in search engines. Subdomains are a lower part of a main domain name, such as "glossary.internet-guide.co.uk" which would mean "glossary" is a subdomain of "http://www.internet-guide.co.uk". At present Internet Guide doesn't use subdomains but you will find plenty of examples of subdomains at other popular UK web sites.