Sub-domains allow you to create second-level domain names under your primary domain name. Sub-domains come in the syntax of anything.yourdomain.com, replacing "www" with with a different sub-domain prefix.

A sub domain allows you to create separate web addresses from just one domain, for example www.shop.com could become www.flower.shop.com. Some hosts set a limit for the number of sub domains you can create.

In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is part of a larger domain name. A DNS hierarchy consists of the root-level domain at the top, underneath which are the top-level domains, followed by second-level domains and finally subdomains.

http://en.wikipedia.org/wiki/Sub_domain