As its name implies, a subdomain is a subsection of a domain. For example, the CDes Web site -- design.umn.edu -- is a subdomain of the University of Minnesota (umn.edu). The College of Design has its own set of subdomains as follows:

Subdomains are appropriate when simplifing management of large domains by splitting logical areas of information or expertise into discrete entities. When considering a subdomain, your primary concern should be the constituent audiences.