A subdomain is the part of the web address that is before a domain name and you have most likely seen a lot of subdomains while surfing around the Internet. For example, many sites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main benefit 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 different server. The practical use is that you could have a supplementary site, like an e-learning portal for pupils in addition to the primary school site. If you work with subdomains as opposed to subfolders, it will be much easier to perform maintenance or to upgrade a certain website, not mentioning that it's going to be more secure to have the websites separate from each other.