A subdomain is the part of the web address which is before a domain and you've most likely seen a lot of subdomains while browsing the Internet. As an example, many sites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. 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 you could have a supplementary website, such as an e-learning portal for college students as well as the primary school website. If you use subdomains as an alternative to subfolders, it will be much easier to perform maintenance or to upgrade a particular site, not mentioning that it's going to be more secure to have the websites separate from each other.