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 exploring world wide web. For example, many websites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The best thing about 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 website, such as an e-learning portal for pupils as well as the main school website. If you are using subdomains instead of subfolders, it's going to be much easier to perform maintenance or to upgrade a specific website, not mentioning that it's going to be more secure to have the sites separate from each other.