A subdomain is the section of the web address that is before a domain and you've quite possibly seen a lot of subdomains while exploring the Web. As an illustration, many websites like Wikipedia have versions in various 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 website and its own records, so you are able to even host it on a different server. The practical use is that one could have a supplementary website, like an e-learning portal for pupils in addition to the main school site. If you work with subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a particular website, not mentioning that it will be more safe to have the websites separate from one another.