A subdomain is the section of the web address that's before a domain and you've probably seen a lot of subdomains while surfing around the Web. As an example, many websites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The best thing about using a subdomain is that it can have a separate website and its own records, so you're able to even host it on another server. The practical use is that one could have a supplementary website, such as an e-learning portal for pupils in addition to 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 website, not mentioning that it'll be more secure to have the websites separate from each other.