A subdomain is the part of the web address that is before a domain and you have probably seen a lot of subdomains while surfing around the Web. As an illustration, many websites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage of using a subdomain is that it can have an independent website and its own records, so you can even host it on a separate server. The practical use is that one could have a supplementary website, like an e-learning portal for students in addition to the main school site. If you use subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a particular website, not mentioning that it's going to be more safe to have the sites separate from one another.