A subdomain is the section of the web address that is before a domain and you've quite possibly seen a lot of subdomains while browsing world wide web. As an example, many sites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of using a subdomain is that it can have an independent website and its own records, so you can even host it on a different server. The practical use is that you can have a supplementary site, such as an e-learning portal for pupils as well as the main school site. If you use subdomains rather than subfolders, it will be much easier to perform maintenance or to upgrade a specific website, not mentioning that it'll be more safe to have the sites separate from each other.