A subdomain is the part of the web address that's before a domain name and you've most likely seen a lot of subdomains while surfing around world wide web. As an example, many websites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The best thing about using a subdomain is that it can have a separate website and its own records, so you can even host it on a different server. The practical use is that you can have a supplementary website, like an e-learning portal for college students aside from the main school website. If you work with subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a specific site, not mentioning that it's going to be more safe to have the sites separate from one another.