A subdomain is the section of the web address that is before a domain and you've almost certainly seen a lot of subdomains while browsing the Web. For example, many websites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of using a subdomain is that it can have an independent website and its own records, so you are able to even host it on a different server. The practical use is that one could have a supplementary site, such as an e-learning portal for college students in addition to the primary school website. If you use subdomains as opposed to subfolders, it will be much easier to perform maintenance or to upgrade a certain website, not mentioning that it'll be more secure to have the websites separate from each other.
