A subdomain is the part of the web address that is before a domain name and you have most likely seen a lot of subdomains while browsing the Internet. For instance, many websites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The best thing about using a subdomain is that it can have an independent website and its own records, so you can even host it on a separate server. The practical use is that one could have a supplementary site, such as an e-learning portal for students aside from the main school site. If you work with subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a certain website, not mentioning that it'll be more safe to have the websites separate from each other.