A subdomain is the part of the web address which is before a domain and you have probably seen a lot of subdomains while browsing world wide web. As an illustration, many sites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of using a subdomain is that it can have an independent website and its own records, so you can even host it on another server. The practical use is that one could have a supplementary site, like an e-learning portal for college students in addition to the main school site. If you work with subdomains as an alternative to subfolders, it will be much easier to perform maintenance or to upgrade a particular website, not mentioning that it'll be more secure to have the websites separate from one another.