A subdomain is the section of the web address that is before a domain and you have most likely seen a lot of subdomains while exploring the Internet. As an example, many sites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main advantage of employing a subdomain is that it can have an independent website 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 site, such as an e-learning portal for students in addition to the main school site. If you work with subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a particular site, not mentioning that it'll be more safe to have the sites separate from one another.