A subdomain is the part of the web address that is before a domain and you've probably seen a lot of subdomains while surfing around world wide web. As an illustration, many websites such as Wikipedia have versions a number of 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're 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 students as well as the main school website. If you work with subdomains instead of subfolders, it will 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 one another.