A subdomain is the section of the web address that's before a domain and you have most likely seen a lot of subdomains while browsing the Internet. For instance, 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 site and its own records, so you can even host it on another server. The practical use is that you can have a supplementary site, like an e-learning portal for students as well as the primary school website. If you work with subdomains rather than subfolders, it will be much easier to perform maintenance or to upgrade a particular site, not mentioning that it will be more secure to have the sites separate from one another.