A subdomain is the section of the web address that is before a domain name and you have quite possibly seen a lot of subdomains while browsing the Web. For instance, many sites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The advantage of using a subdomain is that it can have an independent website and its own records, so you are able to even host it on a different server. The practical use is that you can have a supplementary website, like an e-learning portal for college students in addition to the primary school site. If you use subdomains as opposed to subfolders, it will be much easier to perform maintenance or to upgrade a particular site, not mentioning that it'll be more safe to have the sites separate from one another.