A subdomain is the part of the web address which is before a domain name and you've most likely seen a lot of subdomains while surfing around the Internet. For example, many sites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage of employing a subdomain is that it can have an independent site and its own records, so you can even host it on another 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 website. If you work with subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a specific site, not mentioning that it will be more safe to have the sites separate from each other.