A subdomain is the part of the web address which is before a domain name and you've almost certainly seen a lot of subdomains while browsing the Internet. As an example, many sites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of employing a subdomain is that it can have an independent site and its own records, so you're able to even host it on a separate server. The practical use is that you can have a supplementary website, like an e-learning portal for students aside from the main school website. If you work with subdomains instead of subfolders, it's going to be much easier to perform maintenance or to upgrade a specific site, not mentioning that it's going to be more safe to have the websites separate from one another.