A subdomain is the section of the web address that is before a domain and you've probably seen a lot of subdomains while exploring the Internet. For instance, many websites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The best thing about using a subdomain is that it can have a separate site and its own records, so you are able to even host it on a separate server. The practical use is that one could have a supplementary site, such as an e-learning portal for college students in addition to the primary school website. If you work with subdomains as opposed to subfolders, it'll be much easier to perform maintenance or to upgrade a particular site, not mentioning that it'll be more secure to have the sites separate from one another.