A subdomain is the part of the web address that's before a domain name and you have quite possibly seen a lot of subdomains while surfing around the Web. For instance, many websites like Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The advantage of using a subdomain is that it can have a separate site and its own records, so you are able to even host it on a different server. The practical use is that one could have a supplementary site, such as an e-learning portal for college students aside from the primary school site. If you work with subdomains instead of subfolders, it'll 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 one another.