A subdomain is the part of the web address that is before a domain name and you've most likely seen a lot of subdomains while browsing the Web. For example, many sites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of employing a subdomain is that it can have a separate site and its own records, so you're able to even host it on a separate server. The practical use is that one could have a supplementary website, such as an e-learning portal for college students in addition to the main school website. If you work with subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a particular site, not mentioning that it will be more secure to have the websites separate from one another.