A subdomain is the part of the web address which is before a domain name and you have almost certainly seen a lot of subdomains while browsing the 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 using a subdomain is that it can have an independent website and its own records, so you are able to 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 students as well as the primary school website. If you work with subdomains instead of subfolders, it's going to be much easier to perform maintenance or to upgrade a specific website, not mentioning that it will be more secure to have the sites separate from one another.