A subdomain is the part of the web address that's before a domain name and you have probably seen a lot of subdomains while surfing around the Web. As an example, many websites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The best thing about employing a subdomain is that it can have a separate website and its own records, so you are able to even host it on a different server. The practical use is that one could have a supplementary site, like an e-learning portal for college students in addition to the main school website. If you use subdomains as an alternative to subfolders, it's going to be much easier to perform maintenance or to upgrade a particular website, not mentioning that it will be more safe to have the sites separate from one another.