A subdomain is the section of the web address that is before a domain name and you've quite possibly seen a lot of subdomains while browsing world wide web. For instance, many sites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The advantage of using a subdomain is that it can have a separate site and its own records, so you are able to even host it on a separate server. The practical use is that one could have a supplementary website, like an e-learning portal for pupils as well as the main school site. If you are using subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a particular website, not mentioning that it will be more secure to have the sites separate from one another.