A subdomain is the part of the web address that's before a domain name and you've almost certainly seen a lot of subdomains while surfing around the Web. As an example, many sites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The advantage of employing a subdomain is that it can have an independent website and its own records, so you're able to even host it on a separate server. The practical use is that you could have a supplementary website, such as an e-learning portal for college students aside from the main school website. If you use subdomains as an alternative to subfolders, it will be much easier to perform maintenance or to upgrade a certain website, not mentioning that it's going to be more secure to have the sites separate from each other.