A subdomain is a subdivision of a domain, such as "support.webfusion.com" is a subdomain of "webfusion.com" .
For example:If a customer buys a domain with webfusion, e.g. domain.co.uk, they can set up subdomains, e.g. site1.domain.co.uk or secure.domain.co.uk. This is an excellent way of breaking up the website if you have different regions, products or even languages.