A subdomain is the section of the web address which is before a domain and you have most likely seen a lot of subdomains while exploring the Web. For instance, many websites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage of employing a subdomain is that it can have an independent website and its own records, so you are able to even host it on a separate server. The practical use is that you can have a supplementary website, such as an e-learning portal for students on top of the primary school site. If you are using subdomains as an alternative to subfolders, it will be much easier to perform maintenance or to upgrade a specific site, not mentioning that it will be more secure to have the sites separate from one another.