A subdomain is the section of the web address that's before a domain name and you've almost certainly seen a lot of subdomains while surfing around the Internet. As an illustration, many sites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of using a subdomain is that it can have a separate website and its own records, so you're able to even host it on a different server. The practical use is that you can have a supplementary website, like an e-learning portal for students aside from the main school site. If you work with subdomains as an alternative to subfolders, it will be much easier to perform maintenance or to upgrade a particular website, not mentioning that it'll be more secure to have the websites separate from each other.