A subdomain is the part of the web address that's before a domain name and you've most likely seen a lot of subdomains while exploring the Web. As an example, many sites such as Wikipedia have versions in different 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 you can have a supplementary website, such as an e-learning portal for students aside from 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 site, not mentioning that it will be more safe to have the sites separate from one another.