A subdomain is the section of the web address that's before a domain name and you've most likely seen a lot of subdomains while surfing around the Internet. For instance, many websites such as 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 a separate website 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 pupils aside from the primary 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 certain website, not mentioning that it'll be more safe to have the sites separate from one another.