A subdomain is the part of the web address that is before a domain name and you have almost certainly seen a lot of subdomains while browsing the Internet. As an example, many sites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main benefit 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 you can have a supplementary website, like an e-learning portal for pupils in addition to the main school website. If you work with subdomains as opposed to subfolders, it's going to be much easier to perform maintenance or to upgrade a particular website, not mentioning that it's going to be more secure to have the sites separate from each other.