A subdomain is the section of the web address that is before a domain name and you have most likely seen a lot of subdomains while surfing around the Web. For example, many websites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of employing a subdomain is that it can have an independent website and its own records, so you can even host it on a different server. The practical use is that one could have a supplementary website, like an e-learning portal for pupils on top of the main school site. If you work with subdomains instead of subfolders, it's going to be much easier to perform maintenance or to upgrade a certain website, not mentioning that it'll be more secure to have the sites separate from each other.