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 browsing the Web. For example, many websites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main benefit of employing 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 students on top of the primary school site. If you are using subdomains as an alternative 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 sites separate from one another.