A subdomain is the section of the web address that's before a domain and you've almost certainly seen a lot of subdomains while surfing around the Internet. As an illustration, many websites such as Wikipedia have versions in several 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 a different server. The practical use is that you could have a supplementary site, like an e-learning portal for college students on top of the primary school website. If you use subdomains rather than subfolders, it will be much easier to perform maintenance or to upgrade a specific website, not mentioning that it will be more secure to have the sites separate from one another.