A subdomain is the part of the web address which is before a domain name and you've most likely seen a lot of subdomains while exploring the Web. For example, many websites like 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 can even host it on a separate server. The practical use is that one could have a supplementary website, like an e-learning portal for college students in addition to the main school website. If you are using subdomains as an alternative to subfolders, it'll be much easier to perform maintenance or to upgrade a specific site, not mentioning that it will be more secure to have the websites separate from each other.