A subdomain is the part of the web address that is before a domain and you've almost certainly 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 advantage of employing a subdomain is that it can have a separate site and its own records, so you're able to even host it on a different server. The practical use is that you can have a supplementary site, like an e-learning portal for students in addition to the main school site. If you use subdomains as opposed to subfolders, it will be much easier to perform maintenance or to upgrade a specific site, not mentioning that it's going to be more secure to have the sites separate from one another.