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 browsing the Web. For instance, many sites such as 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're able to even host it on a different 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 use subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a certain site, not mentioning that it will be more secure to have the sites separate from each other.