A subdomain is the part of the web address that is before a domain name and you have probably seen a lot of subdomains while surfing around world wide web. As an example, many websites 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 are able to even host it on another server. The practical use is that one could have a supplementary website, like an e-learning portal for college students in addition to the primary school website. If you are using subdomains as an alternative to subfolders, it's going to be much easier to perform maintenance or to upgrade a specific website, not mentioning that it will be more secure to have the sites separate from one another.