A subdomain is the part of the web address that's before a domain and you've probably seen a lot of subdomains while surfing around the Internet. As an example, many websites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of using a subdomain is that it can have an independent website and its own records, so you are able to even host it on another server. The practical use is that one could have a supplementary website, such as an e-learning portal for pupils on top of the main school site. If you are using subdomains rather than subfolders, it's going to be much easier to perform maintenance or to upgrade a particular site, not mentioning that it's going to be more secure to have the sites separate from one another.