A subdomain is the section of the web address that is before a domain name and you've most likely seen a lot of subdomains while exploring world wide web. For instance, many websites such as Wikipedia have versions a number of 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 are able to even host it on a separate server. The practical use is that you could have a supplementary website, like an e-learning portal for college students as well as 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 certain site, not mentioning that it will be more secure to have the sites separate from each other.