A subdomain is the section of the web address which is before a domain and you have probably seen a lot of subdomains while surfing around the Internet. As an illustration, many sites like Wikipedia have versions in various 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 another 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 primary school site. If you are using subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a specific site, not mentioning that it's going to be more safe to have the websites separate from one another.