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 surfing around the Web. As an example, many sites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The best thing about using a subdomain is that it can have an independent website and its own records, so you're able to even host it on another server. The practical use is that you could have a supplementary website, like an e-learning portal for students as well as the primary school site. If you use subdomains instead of subfolders, it's going to 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.