A subdomain is the part of the web address which is before a domain and you have quite possibly seen a lot of subdomains while browsing the Web. As an example, many sites 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 an independent 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, such as an e-learning portal for students in addition to the main school website. If you work with subdomains as an alternative to subfolders, it will be much easier to perform maintenance or to upgrade a specific site, not mentioning that it'll be more secure to have the websites separate from each other.