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. For instance, many websites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The best thing about using a subdomain is that it can have a separate site and its own records, so you are able to even host it on a separate server. The practical use is that you can have a supplementary website, like an e-learning portal for students on top of the primary school site. If you are using subdomains as an alternative to subfolders, it will be much easier to perform maintenance or to upgrade a specific website, not mentioning that it will be more safe to have the sites separate from each other.