A subdomain is the part of the web address that is before a domain name and you've almost certainly seen a lot of subdomains while surfing around world wide web. For instance, many sites such as Wikipedia have versions in different 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 can even host it on another server. The practical use is that one could have a supplementary website, such as an e-learning portal for pupils in addition to the primary school website. If you work with subdomains rather than subfolders, it's going to be much easier to perform maintenance or to upgrade a certain site, not mentioning that it'll be more safe to have the sites separate from each other.