A subdomain is the part of the web address that is before a domain and you have most likely seen a lot of subdomains while surfing around world wide web. For instance, many websites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main benefit of using a subdomain is that it can have an independent site and its own records, so you are able to even host it on a separate server. The practical use is that one could have a supplementary site, such as an e-learning portal for pupils aside from the main school website. If you are using subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a particular website, not mentioning that it's going to be more safe to have the sites separate from one another.