A subdomain is the section of the web address that is before a domain name and you have quite possibly seen a lot of subdomains while browsing the Web. As an illustration, many sites such as Wikipedia have versions in several 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 site and its own records, so you can even host it on a different server. The practical use is that you could have a supplementary site, like an e-learning portal for students in addition to the primary school site. If you are using subdomains as opposed to subfolders, it's going to be much easier to perform maintenance or to upgrade a specific site, not mentioning that it's going to be more safe to have the websites separate from one another.