A subdomain is the section of the web address that is before a domain name and you have almost certainly seen a lot of subdomains while surfing around the Internet. For instance, many sites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of using a subdomain is that it can have a separate 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 site, like an e-learning portal for pupils aside from the primary school website. If you work with subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a specific website, not mentioning that it'll be more safe to have the websites separate from one another.