A subdomain is the part of the web address that's before a domain and you have probably seen a lot of subdomains while exploring the Web. As an example, many sites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. 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 another server. The practical use is that one could have a supplementary site, like an e-learning portal for students as well as the main school website. If you use subdomains as opposed to subfolders, it's going to be much easier to perform maintenance or to upgrade a specific site, not mentioning that it'll be more safe to have the sites separate from each other.