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 such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The advantage of using a subdomain is that it can have a separate site and its own records, so you're able to even host it on a different server. The practical use is that you can have a supplementary website, like an e-learning portal for pupils aside from the primary school website. If you use subdomains as opposed to subfolders, it'll 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 one another.