A subdomain is the part of the web address that's before a domain name and you've probably seen a lot of subdomains while surfing around the Web. As an example, many sites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The advantage of using a subdomain is that it can have an independent site and its own records, so you can even host it on a separate server. The practical use is that you 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 as an alternative to subfolders, it's going to be much easier to perform maintenance or to upgrade a particular website, not mentioning that it'll be more secure to have the websites separate from each other.