A subdomain is the section of the web address which is before a domain name and you have most likely seen a lot of subdomains while exploring world wide web. For example, many sites such as Wikipedia have versions in several 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 another server. The practical use is that you can have a supplementary site, like an e-learning portal for college students in addition to the primary school site. If you work with subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a particular website, not mentioning that it's going to be more safe to have the websites separate from one another.