A subdomain is the part of the web address that's before a domain and you've quite possibly seen a lot of subdomains while exploring the Web. For instance, many websites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main benefit of using a subdomain is that it can have an independent website and its own records, so you can even host it on a different 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 website. If you use subdomains as opposed to subfolders, it will be much easier to perform maintenance or to upgrade a certain site, not mentioning that it's going to be more safe to have the sites separate from each other.