A subdomain is the section of the web address which is before a domain and you have probably seen a lot of subdomains while browsing the Web. As an illustration, many sites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The best thing about using a subdomain is that it can have a separate website and its own records, so you're able to even host it on another server. The practical use is that you could have a supplementary website, such as an e-learning portal for college students in addition to the primary school website. If you use subdomains instead of 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.