A subdomain is the section of the web address which is before a domain and you've probably seen a lot of subdomains while exploring the Web. As an example, many websites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage of using a subdomain is that it can have a separate site and its own records, so you can even host it on a different server. The practical use is that you can have a supplementary website, like an e-learning portal for college students in addition to the main school site. If you use subdomains rather than subfolders, it's going to be much easier to perform maintenance or to upgrade a certain site, not mentioning that it's going to be more secure to have the websites separate from each other.