A subdomain is the section of the web address which is before a domain and you've almost certainly seen a lot of subdomains while exploring the Internet. As an illustration, many websites such as Wikipedia have versions in various 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 can even host it on a separate 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 are using subdomains as opposed to 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 websites separate from one another.