A subdomain is the part of the web address which is before a domain and you have quite possibly seen a lot of subdomains while exploring world wide web. As an illustration, many sites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage of using a subdomain is that it can have a separate website and its own records, so you're able to even host it on a different server. The practical use is that you can have a supplementary website, such as an e-learning portal for pupils as well as the primary school site. If you use subdomains rather than 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 websites separate from each other.