A subdomain is the part of the web address which is before a domain name and you've quite possibly seen a lot of subdomains while browsing the Web. For example, many sites like 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 site 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, such as an e-learning portal for students aside from the main school site. If you are using subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a particular website, not mentioning that it will be more safe to have the sites separate from each other.