A subdomain is the section of the web address which is before a domain name and you've most likely seen a lot of subdomains while surfing around the Web. As an example, many websites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main benefit of employing a subdomain is that it can have a separate website and its own records, so you are able to even host it on a separate server. The practical use is that one could have a supplementary site, like 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 website, not mentioning that it's going to be more secure to have the websites separate from one another.