A subdomain is the part of the web address which is before a domain name and you've most likely seen a lot of subdomains while surfing around world wide web. For instance, many websites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. 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 another server. The practical use is that you could have a supplementary site, such as an e-learning portal for college students aside from the primary school site. If you are using subdomains as opposed to subfolders, it will be much easier to perform maintenance or to upgrade a specific website, not mentioning that it'll be more secure to have the websites separate from each other.