A subdomain is the part of the web address that is before a domain name and you have quite possibly seen a lot of subdomains while surfing around the Web. For example, many websites like Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of employing a subdomain is that it can have a separate site and its own records, so you're able to even host it on a different server. The practical use is that you could have a supplementary site, like an e-learning portal for pupils on top of the primary school website. If you work with subdomains rather than subfolders, it will be much easier to perform maintenance or to upgrade a certain site, not mentioning that it will be more safe to have the sites separate from one another.