A subdomain is the part of the web address which is before a domain name and you've quite possibly seen a lot of subdomains while browsing world wide web. As an example, many sites like Wikipedia have versions in several 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 website and its own records, so you're able to even host it on a different server. The practical use is that you could have a supplementary site, like an e-learning portal for pupils in addition to the primary school website. If you work with subdomains as an alternative to subfolders, it will be much easier to perform maintenance or to upgrade a specific site, not mentioning that it'll be more secure to have the websites separate from each other.