A subdomain is the section of the web address that's before a domain name and you have most likely seen a lot of subdomains while surfing around world wide web. For instance, many websites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The best thing about employing 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 primary school website. If you use subdomains instead of subfolders, it's going to 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 each other.