A subdomain is the section of the web address that's before a domain and you have most likely seen a lot of subdomains while surfing around world wide web. For example, many websites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of using a subdomain is that it can have an independent site and its own records, so you're able to even host it on a separate server. The practical use is that you can have a supplementary site, such as an e-learning portal for pupils aside from the main 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 websites separate from one another.