A subdomain is the part of the web address that's before a domain name and you have almost certainly seen a lot of subdomains while surfing around world wide web. For instance, many websites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. 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 another server. The practical use is that one could have a supplementary website, such as an e-learning portal for college students in addition to the primary school website. If you use subdomains instead of subfolders, it's going to be much easier to perform maintenance or to upgrade a particular site, not mentioning that it will be more safe to have the sites separate from each other.