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 exploring the Internet. For example, many websites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of employing a subdomain is that it can have an independent site and its own records, so you are able to even host it on another server. The practical use is that you could have a supplementary website, such as an e-learning portal for pupils as well as the main school website. If you work with subdomains rather than subfolders, it's going to be much easier to perform maintenance or to upgrade a specific site, not mentioning that it will be more secure to have the websites separate from each other.