A subdomain is the part of the web address which is before a domain and you have quite possibly seen a lot of subdomains while exploring the Web. As an example, many sites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The best thing about employing a subdomain is that it can have an independent website and its own records, so you're able to even host it on another server. The practical use is that you can have a supplementary website, like an e-learning portal for students as well as the main school site. If you work with subdomains rather than subfolders, it's going to be much easier to perform maintenance or to upgrade a certain website, not mentioning that it will be more safe to have the websites separate from each other.