A subdomain is the section of the web address that's before a domain and you've probably seen a lot of subdomains while exploring the Internet. As an illustration, many websites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage of using 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 one could have a supplementary site, such as an e-learning portal for students in addition to the main school website. If you work with subdomains as opposed to subfolders, it's going to be much easier to perform maintenance or to upgrade a certain site, not mentioning that it will be more safe to have the sites separate from one another.