A subdomain is the part of the web address that's before a domain name and you have quite possibly seen a lot of subdomains while exploring the Internet. For instance, many websites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main benefit of employing a subdomain is that it can have a separate website and its own records, so you are able to even host it on a different server. The practical use is that one could have a supplementary site, like an e-learning portal for college students in addition to the primary school site. If you work with subdomains as opposed to subfolders, it will be much easier to perform maintenance or to upgrade a specific site, not mentioning that it's going to be more safe to have the websites separate from one another.