A subdomain is the part of the web address which is before a domain and you have most likely seen a lot of subdomains while surfing around the Internet. As an example, many websites like Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The best thing about employing a subdomain is that it can have an independent site and its own records, so you are able to even host it on a separate server. The practical use is that one could have a supplementary website, such as an e-learning portal for pupils as well as the main school site. If you work with subdomains rather than subfolders, it's going to be much easier to perform maintenance or to upgrade a particular site, not mentioning that it'll be more secure to have the websites separate from one another.