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 world wide web. For example, many sites like 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 site, like an e-learning portal for students as well as the primary school website. If you work with subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a specific website, not mentioning that it'll be more safe to have the sites separate from each other.