A subdomain is the section of the web address which is before a domain and you have 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 etc. The best thing about using a subdomain is that it can have a separate 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, like an e-learning portal for pupils aside from the primary school site. If you use subdomains instead of 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 websites separate from each other.