A subdomain is the part of the web address that is before a domain name and you have most likely seen a lot of subdomains while surfing around world wide web. For example, many websites such as 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 a different server. The practical use is that one could have a supplementary site, such as an e-learning portal for pupils in addition to the primary school site. If you work with subdomains as opposed to subfolders, it's going to be much easier to perform maintenance or to upgrade a certain website, not mentioning that it will be more safe to have the sites separate from each other.