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 exploring world wide web. As an illustration, many sites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The best thing about 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 separate server. The practical use is that you could have a supplementary website, such as an e-learning portal for college students in addition to the main school site. If you are using subdomains instead of 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 websites separate from each other.