A subdomain is the section of the web address that is before a domain name and you've quite possibly seen a lot of subdomains while browsing world wide web. For example, many websites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage of employing a subdomain is that it can have an independent site and its own records, so you're able to even host it on a different server. The practical use is that one could have a supplementary website, like an e-learning portal for college students as well as the primary school website. If you work with subdomains as opposed to subfolders, it'll be much easier to perform maintenance or to upgrade a particular site, not mentioning that it will be more secure to have the sites separate from each other.