A subdomain is the section of the web address which is before a domain name and you have almost certainly seen a lot of subdomains while browsing the Web. As an example, many sites like Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage of using a subdomain is that it can have a separate website 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 students as well as the main 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 will be more secure to have the websites separate from one another.