A subdomain is the part of the web address that's before a domain and you have almost certainly seen a lot of subdomains while browsing 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 main benefit of employing a subdomain is that it can have a separate 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 site, like an e-learning portal for college students on top of the main school site. 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's going to be more safe to have the websites separate from one another.