A subdomain is the part of the web address that's before a domain name and you've probably seen a lot of subdomains while browsing the Web. As an example, many sites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main benefit of using a subdomain is that it can have a separate website and its own records, so you can even host it on another server. The practical use is that you can have a supplementary website, such as an e-learning portal for college students on top of the main school site. If you work with subdomains as opposed to subfolders, it'll be much easier to perform maintenance or to upgrade a certain site, not mentioning that it will be more secure to have the websites separate from each other.