A subdomain is the section of the web address which is before a domain and you've almost certainly seen a lot of subdomains while browsing the Internet. As an example, many websites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The best thing about employing a subdomain is that it can have a separate website and its own records, so you're able to even host it on a different server. The practical use is that you could have a supplementary site, such as an e-learning portal for college students aside from the main school website. If you are using subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a specific website, not mentioning that it will be more safe to have the websites separate from one another.