In DNS hierarchy, a subdomain is a domain that is part of a larger domain name. 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.

For example, in the domain name msdn.microsoft.com, "msdn" is a subdomain of the larger second-level domain "Microsoft.com."

msdn.microsoft.com - msdn is a sub domain.

You can automatically create, delete and modify sub domains right from the Server Intellect control panel.