A subdomain is the section of the web address which is before a domain and you have most likely seen a lot of subdomains while surfing around the Internet. As an example, many websites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage of using a subdomain is that it can have an independent site and its own records, so you can even host it on another server. The practical use is that one could have a supplementary website, such as an e-learning portal for college students in addition to the primary school site. If you work with subdomains rather than subfolders, it will be much easier to perform maintenance or to upgrade a specific site, not mentioning that it will be more safe to have the websites separate from each other.