A subdomain is the section of the web address that is before a domain name and you have quite possibly seen a lot of subdomains while surfing around the Web. As an example, many websites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage 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 another server. The practical use is that one could have a supplementary site, like an e-learning portal for students aside from the primary school website. If you are using subdomains as opposed to subfolders, it'll be much easier to perform maintenance or to upgrade a particular website, not mentioning that it will be more secure to have the websites separate from one another.