A subdomain is the section of the web address which is before a domain name and you've probably seen a lot of subdomains while surfing around the Internet. For example, many sites 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 an independent site and its own records, so you are able to even host it on a different server. The practical use is that you can have a supplementary site, like an e-learning portal for pupils on top of the main school site. If you use subdomains as an alternative to subfolders, it will be much easier to perform maintenance or to upgrade a certain site, not mentioning that it will be more secure to have the sites separate from one another.