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 surfing around 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 on. The main advantage 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 separate server. The practical use is that you can have a supplementary site, like an e-learning portal for students as well as the primary school website. If you work with subdomains rather than subfolders, it'll 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 websites separate from one another.