A subdomain is the part of the web address that is before a domain name and you have probably seen a lot of subdomains while exploring the Web. As an example, many sites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The advantage of using a subdomain is that it can have a separate site and its own records, so you can 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 aside from the main school website. If you are using subdomains as opposed to subfolders, it will be much easier to perform maintenance or to upgrade a particular site, not mentioning that it's going to be more safe to have the websites separate from each other.