A subdomain is the part of the web address that's before a domain name and you have almost certainly seen a lot of subdomains while surfing around world wide web. For instance, many sites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage of using a subdomain is that it can have a separate website and its own records, so you can even host it on a separate server. The practical use is that you can have a supplementary website, like an e-learning portal for pupils aside from the main school site. If you use subdomains as opposed to subfolders, it'll be much easier to perform maintenance or to upgrade a specific site, not mentioning that it'll be more secure to have the sites separate from one another.