A subdomain is the section of the web address that's before a domain name and you've almost certainly seen a lot of subdomains while surfing around the Web. As an example, many websites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main benefit of employing a subdomain is that it can have a separate site and its own records, so you can even host it on a different server. The practical use is that one could have a supplementary website, such as an e-learning portal for college students on top of 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 particular site, not mentioning that it's going to be more secure to have the sites separate from each other.