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 exploring the Web. For instance, many websites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of employing 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 could have a supplementary site, like an e-learning portal for pupils aside from the primary school website. If you are using subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a certain website, not mentioning that it will be more safe to have the sites separate from one another.