A subdomain is the part of the web address which is before a domain and you have quite possibly seen a lot of subdomains while exploring the Web. As an example, many sites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. 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 website, like an e-learning portal for students on top of the main school website. If you are using subdomains as opposed to subfolders, it'll be much easier to perform maintenance or to upgrade a particular site, not mentioning that it will be more secure to have the websites separate from one another.