A subdomain is the section of the web address which is before a domain and you have almost certainly seen a lot of subdomains while exploring the Web. As an example, many websites such as Wikipedia have versions in various 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 site and its own records, so you're able to even host it on a different server. The practical use is that you could have a supplementary site, such as an e-learning portal for college students aside from the primary 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's going to be more secure to have the sites separate from one another.