A subdomain is the section of the web address which is before a domain name and you've almost certainly seen a lot of subdomains while exploring the Web. For example, many sites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The best thing about using a subdomain is that it can have an independent website and its own records, so you are able to even host it on a different server. The practical use is that you could have a supplementary website, such as an e-learning portal for college students in addition to the main school site. If you use 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 will be more secure to have the websites separate from one another.