A subdomain is the section of the web address which is before a domain and you've probably seen a lot of subdomains while surfing around the Internet. For example, many websites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of employing a subdomain is that it can have an independent website and its own records, so you're able to even host it on a separate server. The practical use is that you can have a supplementary website, such as an e-learning portal for college students in addition to the main 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'll be more safe to have the sites separate from each other.