A subdomain is the part of the web address that's before a domain and you've quite possibly seen a lot of subdomains while browsing world wide web. As an illustration, many websites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The best thing about 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 can have a supplementary site, like an e-learning portal for pupils as well as the primary school website. If you are using subdomains as an alternative to subfolders, it'll be much easier to perform maintenance or to upgrade a specific site, not mentioning that it's going to be more secure to have the sites separate from one another.