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