A subdomain is the section of the web address which is before a domain name and you have almost certainly 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 main advantage of employing 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 you can have a supplementary website, such as an e-learning portal for students aside from the primary school site. If you use subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a specific website, not mentioning that it'll be more safe to have the sites separate from one another.