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 exploring the Internet. For example, many sites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of using a subdomain is that it can have an independent website and its own records, so you can even host it on a different server. The practical use is that you can have a supplementary site, like an e-learning portal for pupils as well as the primary school site. If you are using subdomains as opposed to subfolders, it'll be much easier to perform maintenance or to upgrade a specific website, not mentioning that it will be more safe to have the websites separate from one another.
