A subdomain is the section of the web address which is before a domain name and you've quite possibly seen a lot of subdomains while browsing the Web. For example, many websites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The best thing about using a subdomain is that it can have an independent website and its own records, so you are able to even host it on another server. The practical use is that one could have a supplementary website, such as an e-learning portal for college students aside from the primary school site. If you are using subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a specific site, not mentioning that it will be more secure to have the sites separate from one another.