A subdomain is the part of the web address which is before a domain and you've most likely seen a lot of subdomains while surfing around world wide web. As an illustration, many sites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main benefit of employing a subdomain is that it can have a separate site and its own records, so you can even host it on another server. The practical use is that you can have a supplementary site, like an e-learning portal for college students as well as the main school site. If you use subdomains rather than subfolders, it will be much easier to perform maintenance or to upgrade a specific site, not mentioning that it's going to be more secure to have the websites separate from each other.