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 surfing around the Internet. As an example, many sites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The best thing about employing a subdomain is that it can have a separate website and its own records, so you can even host it on another server. The practical use is that one could have a supplementary site, like an e-learning portal for students aside from the main school site. If you use subdomains as an alternative to 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.