A subdomain is the section of the web address that's before a domain and you have almost certainly seen a lot of subdomains while surfing around the Web. For example, many sites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. 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 another server. The practical use is that you can 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's going to be much easier to perform maintenance or to upgrade a specific site, not mentioning that it's going to be more secure to have the sites separate from one another.