A subdomain is the part of the web address which is before a domain and you have almost certainly seen a lot of subdomains while browsing world wide web. As an illustration, many sites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The advantage of employing 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, such as an e-learning portal for students as well as the main school site. If you are using subdomains as opposed to subfolders, it's going to be much easier to perform maintenance or to upgrade a certain website, not mentioning that it's going to be more safe to have the sites separate from each other.