A subdomain is the section of the web address that is before a domain name and you've almost certainly seen a lot of subdomains while exploring world wide web. As an example, many websites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main benefit of employing a subdomain is that it can have an independent site and its own records, so you can even host it on a separate server. The practical use is that one could have a supplementary site, such as an e-learning portal for pupils aside from the main school site. If you use subdomains as opposed to subfolders, it'll be much easier to perform maintenance or to upgrade a particular site, not mentioning that it's going to be more secure to have the sites separate from one another.