A subdomain is the section of the web address that is before a domain and you have quite possibly seen a lot of subdomains while browsing the Web. As an example, many sites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The advantage of employing a subdomain is that it can have an independent site and its own records, so you are able to even host it on another server. The practical use is that you can have a supplementary website, such as an e-learning portal for college students as well as the main school site. 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 will be more safe to have the websites separate from each other.