A subdomain is the section of the web address that's before a domain name and you've almost certainly seen a lot of subdomains while browsing world wide web. For example, many sites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage of using a subdomain is that it can have a separate site and its own records, so you are able to even host it on a different server. The practical use is that you could have a supplementary site, like an e-learning portal for college students in addition to the primary school website. If you are using subdomains as an alternative to subfolders, it will be much easier to perform maintenance or to upgrade a particular website, not mentioning that it will be more secure to have the sites separate from one another.