A subdomain is the section of the web address that is before a domain name and you have quite possibly seen a lot of subdomains while browsing the Internet. For instance, many sites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main benefit of employing a subdomain is that it can have an independent site and its own records, so you're able to even host it on another server. The practical use is that one could have a supplementary site, like an e-learning portal for pupils in addition to the primary school site. If you use subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a particular site, not mentioning that it will be more secure to have the websites separate from each other.