A subdomain is the part of the web address which is before a domain and you've quite possibly seen a lot of subdomains while browsing the Web. For example, many sites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of using a subdomain is that it can have an independent site and its own records, so you are able to even host it on a different server. The practical use is that you can have a supplementary site, like an e-learning portal for pupils in addition to the main school website. If you are using subdomains as an alternative to subfolders, it'll be much easier to perform maintenance or to upgrade a specific website, not mentioning that it will be more secure to have the websites separate from one another.