A subdomain is the part of the web address that's before a domain and you've most likely seen a lot of subdomains while exploring the Web. As an illustration, many websites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of employing a subdomain is that it can have a separate website and its own records, so you can even host it on a different server. The practical use is that one could have a supplementary website, such as an e-learning portal for pupils in addition to the main school website. If you use subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a particular site, not mentioning that it's going to be more secure to have the websites separate from each other.