A subdomain is the section of the web address that is before a domain and you've most likely seen a lot of subdomains while exploring the Internet. For example, many websites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of using a subdomain is that it can have a separate site and its own records, so you are 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 pupils as well as the primary school site. If you work with subdomains as an alternative to subfolders, it will be much easier to perform maintenance or to upgrade a particular site, not mentioning that it will be more secure to have the sites separate from one another.