A subdomain is the section of the web address that's before a domain name and you've almost certainly seen a lot of subdomains while browsing world wide web. For example, many websites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of using a subdomain is that it can have an independent site and its own records, so you are able to even host it on a separate server. The practical use is that you could have a supplementary site, like an e-learning portal for students as well as the main school site. If you are using subdomains rather than subfolders, it will be much easier to perform maintenance or to upgrade a particular website, not mentioning that it's going to be more safe to have the sites separate from one another.