A subdomain is the section of the web address which is before a domain name and you have quite possibly seen a lot of subdomains while surfing around world wide web. As an example, many websites like Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The best thing about employing a subdomain is that it can have an independent site and its own records, so you're able to even host it on a different server. The practical use is that one could have a supplementary website, such as an e-learning portal for students aside from the main school site. If you work with subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a specific site, not mentioning that it'll be more safe to have the sites separate from one another.