A subdomain is the section of the web address that is before a domain name and you have probably seen a lot of subdomains while browsing the Internet. As an illustration, many sites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The best thing about employing a subdomain is that it can have an independent website 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 site, like an e-learning portal for students in addition to the main school site. If you are using subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a specific site, not mentioning that it will be more secure to have the sites separate from one another.
