A subdomain is the part of the web address which is before a domain name and you have almost certainly seen a lot of subdomains while exploring the Web. As an example, many websites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The best thing about 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 separate server. The practical use is that one could have a supplementary website, like an e-learning portal for college students on top of the primary school website. If you are using subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a specific site, not mentioning that it'll be more secure to have the websites separate from each other.