A subdomain is the part of the web address which is before a domain name and you've almost certainly seen a lot of subdomains while surfing around world wide web. As an illustration, many sites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main benefit 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 separate server. The practical use is that one could have a supplementary site, such as an e-learning portal for college students aside from the primary school site. If you use subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a certain site, not mentioning that it's going to be more safe to have the sites separate from each other.