A subdomain is the part of the web address that is before a domain and you've probably seen a lot of subdomains while browsing world wide web. As an illustration, many sites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main benefit of employing a subdomain is that it can have a separate site 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, like an e-learning portal for college students on top of the primary school website. If you work with subdomains as opposed to subfolders, it'll be much easier to perform maintenance or to upgrade a particular site, not mentioning that it'll be more secure to have the sites separate from each other.