A subdomain is the section of the web address that is before a domain and you have quite possibly seen a lot of subdomains while surfing around world wide web. For example, many sites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main advantage of using a subdomain is that it can have an independent site and its own records, so you can even host it on a different server. The practical use is that one could have a supplementary website, such as an e-learning portal for students in addition to the primary school site. If you use subdomains as an alternative to subfolders, it'll be much easier to perform maintenance or to upgrade a particular site, not mentioning that it'll be more secure to have the sites separate from each other.