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. As an illustration, many websites like Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The advantage of using a subdomain is that it can have an independent website and its own records, so you can even host it on another server. The practical use is that you can have a supplementary website, like an e-learning portal for college students in addition to the main school website. If you work with subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a certain website, not mentioning that it's going to be more safe to have the sites separate from each other.