A subdomain is the section of the web address which is before a domain and you've quite possibly seen a lot of subdomains while exploring world wide web. As an example, many websites like Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The best thing about using a subdomain is that it can have an independent site and its own records, so you're able to even host it on a separate server. The practical use is that you could have a supplementary site, such as an e-learning portal for students aside from the main school website. If you work with subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a certain website, not mentioning that it'll be more secure to have the sites separate from one another.