A subdomain is the section of the web address that's before a domain name and you've quite possibly seen a lot of subdomains while exploring the Web. As an illustration, many sites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The advantage of employing a subdomain is that it can have an independent site and its own records, so you are able to even host it on a different server. The practical use is that one could have a supplementary site, such as an e-learning portal for pupils 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 particular website, not mentioning that it will be more safe to have the websites separate from one another.