A subdomain is the part of the web address which is before a domain name and you have probably seen a lot of subdomains while browsing the Web. As an illustration, many websites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The best thing about employing a subdomain is that it can have a separate website and its own records, so you are able to even host it on another server. The practical use is that one could have a supplementary website, such as an e-learning portal for college students in addition to the primary school site. If you use subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a specific website, not mentioning that it will be more secure to have the sites separate from each other.