A subdomain is the section of the web address that's before a domain and you've most likely seen a lot of subdomains while browsing world wide web. As an example, many sites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main benefit of employing a subdomain is that it can have an independent website and its own records, so you're able to even host it on a separate server. The practical use is that you could have a supplementary website, like an e-learning portal for college students in addition to the main school site. If you use subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a certain site, not mentioning that it will be more secure to have the websites separate from one another.