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