A subdomain is the part of the web address that is before a domain name and you have almost certainly seen a lot of subdomains while exploring the Internet. As an example, many sites such as Wikipedia have versions a number of 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 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, like an e-learning portal for college students in addition to the main school website. If you are using subdomains rather than subfolders, it's going to 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 each other.