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. For example, many sites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of employing 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 site, such as an e-learning portal for college students on top of the main school site. If you work with subdomains rather than subfolders, it's going to be much easier to perform maintenance or to upgrade a particular website, not mentioning that it's going to be more secure to have the sites separate from each other.