A subdomain is the section of the web address that is before a domain name and you've most likely seen a lot of subdomains while surfing around the Web. As an example, many sites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The best thing about using a subdomain is that it can have an independent website and its own records, so you can 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 students aside from the main school website. If you work with subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a certain website, not mentioning that it's going to be more safe to have the sites separate from one another.