A subdomain is the part of the web address which is before a domain name and you have most likely seen a lot of subdomains while exploring the Web. For instance, many sites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The 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 different server. The practical use is that you can have a supplementary website, like an e-learning portal for pupils in addition to the primary school website. 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'll be more safe to have the sites separate from one another.