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