A subdomain is the section of the web address which is before a domain and you've almost certainly seen a lot of subdomains while surfing around the Web. For example, many websites 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 an independent site and its own records, so you are able to even host it on a different server. The practical use is that you can have a supplementary site, like an e-learning portal for pupils as well as the primary school site. If you are using subdomains as an alternative to subfolders, it'll be much easier to perform maintenance or to upgrade a particular site, not mentioning that it's going to be more safe to have the websites separate from each other.