A subdomain is the section of the web address which is before a domain and you've quite possibly seen a lot of subdomains while surfing around world wide web. For instance, many sites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The best thing about employing a subdomain is that it can have an independent website and its own records, so you are able to even host it on a separate server. The practical use is that you can have a supplementary site, such as an e-learning portal for students as well as the primary school website. If you work with subdomains as an alternative to subfolders, it'll be much easier to perform maintenance or to upgrade a particular website, not mentioning that it'll be more secure to have the sites separate from each other.