A subdomain is the part of the web address that is before a domain name and you've most likely seen a lot of subdomains while surfing around the Internet. As an example, many websites like Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of using a subdomain is that it can have an independent site and its own records, so you are able to even host it on another server. The practical use is that you could have a supplementary site, like an e-learning portal for college students as well as the main school site. If you are using subdomains as an alternative to subfolders, it will be much easier to perform maintenance or to upgrade a particular website, not mentioning that it'll be more safe to have the sites separate from each other.