A subdomain is the part of the web address that's before a domain and you've almost certainly seen a lot of subdomains while exploring the Internet. For example, many websites like Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main benefit of employing a subdomain is that it can have an independent site and its own records, so you're able to even host it on another server. The practical use is that you could have a supplementary website, like an e-learning portal for college students 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 site, not mentioning that it will be more secure to have the sites separate from one another.