A subdomain is a domain defined within a higher level domain space. A subdomain can be referenced and used just the same as the domain that it is subordinate to. There are a variety of reasons why you would create subdomains but they are usually used for two purposes.
1. To separate the site into logical subsections.
2. To divide the site into sections that are maintained by different people.
For example, my computer help site has the domain name of felgall.com. I have some pages where I offer a web site construction award that is in its own subdomain at award.felgall.com and I have some pages about model railways at rail.felgall.com. I look after one of the 500 subjects for about.com and so I have access to javascript.about.com but not any of the rest of the domain. Similarly anyone sighning up for free hosting at Bravenet will get their own subdomain within bravehost.com but will not have access to any other subdomains.
