A subdomain is the part of the web address which is before a domain and you've most likely seen a lot of subdomains while browsing the Web. As an illustration, many websites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of using a subdomain is that it can have a separate site 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 site, such as an e-learning portal for college students on top of 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 particular website, not mentioning that it'll be more secure to have the sites separate from each other.