A subdomain is the section of the web address which is before a domain and you have quite possibly seen a lot of subdomains while exploring world wide web. For instance, 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 are able to even host it on another server. The practical use is that you could have a supplementary website, like an e-learning portal for students on top of the main school website. If you use subdomains as an alternative to subfolders, it'll be much easier to perform maintenance or to upgrade a specific website, not mentioning that it'll be more secure to have the websites separate from each other.