A subdomain is the section of the web address that's before a domain name and you have most likely seen a lot of subdomains while surfing around world wide web. For instance, many websites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of using a subdomain is that it can have a separate site and its own records, so you can even host it on a separate server. The practical use is that one could have a supplementary site, like an e-learning portal for students on top of the primary school site. If you are using subdomains as an alternative to subfolders, it will be much easier to perform maintenance or to upgrade a certain website, not mentioning that it will be more secure to have the websites separate from each other.