A subdomain is the section of the web address that's before a domain and you have probably seen a lot of subdomains while surfing around world wide web. As an illustration, many websites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. 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 separate server. The practical use is that you can have a supplementary site, like an e-learning portal for college students in addition to the main school site. If you are using 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'll be more secure to have the websites separate from each other.