A subdomain is the part of the web address that is before a domain name and you've quite possibly seen a lot of subdomains while surfing around the Internet. As an illustration, many websites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of employing a subdomain is that it can have a separate website and its own records, so you are able to even host it on another server. The practical use is that you can have a supplementary site, such as an e-learning portal for students as well as the primary school site. If you use subdomains instead of subfolders, it's going to be much easier to perform maintenance or to upgrade a specific site, not mentioning that it will be more safe to have the sites separate from one another.