A subdomain is the part of the web address which is before a domain and you have quite possibly seen a lot of subdomains while surfing around the Internet. For instance, many websites such as 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 a separate website 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 website, such as an e-learning portal for college students as well as the main school website. If you use subdomains as opposed to subfolders, it's going to be much easier to perform maintenance or to upgrade a specific website, not mentioning that it's going to be more secure to have the sites separate from one another.