A subdomain is the section of the web address which is before a domain and you've probably seen a lot of subdomains while browsing the Internet. As an illustration, many websites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of using a subdomain is that it can have an independent website and its own records, so you're able to even host it on a different server. The practical use is that one could have a supplementary site, like an e-learning portal for college students as well as the main school site. If you work with 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 secure to have the sites separate from one another.