A subdomain is the part of the web address that's before a domain name and you've quite possibly seen a lot of subdomains while exploring the Web. For instance, many websites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The best thing about 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 separate server. The practical use is that one could have a supplementary site, like an e-learning portal for college students on top of the main school site. If you are using subdomains rather than subfolders, it's going to be much easier to perform maintenance or to upgrade a specific site, not mentioning that it will be more safe to have the websites separate from one another.