A subdomain is the section of the web address that's before a domain name and you've almost certainly seen a lot of subdomains while surfing around world wide web. For instance, many websites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of employing a subdomain is that it can have a separate site and its own records, so you can even host it on a separate server. The practical use is that you can have a supplementary website, such as an e-learning portal for college students as well as the main school site. If you are using subdomains instead of subfolders, it's going to be much easier to perform maintenance or to upgrade a specific site, not mentioning that it's going to be more safe to have the websites separate from each other.