A subdomain is the part of the web address which is before a domain and you have quite possibly seen a lot of subdomains while browsing 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 on. The main benefit 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 separate server. The practical use is that one could have a supplementary website, such as an e-learning portal for students in addition to the main school website. If you use subdomains as an alternative to subfolders, it's going to be much easier to perform maintenance or to upgrade a certain site, not mentioning that it will be more safe to have the websites separate from one another.