A subdomain is the section of the web address which is before a domain and you've probably seen a lot of subdomains while surfing around the Web. As an illustration, many sites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main advantage of using a subdomain is that it can have a separate site 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 main school website. If you use subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a specific website, not mentioning that it's going to be more secure to have the sites separate from each other.