A subdomain is the section of the web address that is before a domain and you have 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 forth. The main benefit of using a subdomain is that it can have an independent site and its own records, so you are 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 as well as the primary school website. If you work with subdomains as an alternative to subfolders, it's going to 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 sites separate from one another.