A subdomain is the part of the web address that's before a domain and you've most likely seen a lot of subdomains while surfing around the Internet. For instance, many sites like Wikipedia have versions a number of 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 site and its own records, so you can even host it on a different server. The practical use is that you can have a supplementary website, such as an e-learning portal for pupils aside from the main school website. If you work with 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.