A subdomain is the part of the web address that's before a domain and you've almost certainly seen a lot of subdomains while exploring the Web. For instance, many sites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage of using a subdomain is that it can have a separate site 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 site, like an e-learning portal for college students in addition to the main school site. If you use subdomains as an alternative to subfolders, it will be much easier to perform maintenance or to upgrade a specific site, not mentioning that it'll be more secure to have the websites separate from one another.