A subdomain is the part of the web address that is before a domain and you've most likely seen a lot of subdomains while surfing around the Internet. For example, many sites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main benefit of using a subdomain is that it can have an independent site and its own records, so you're able to even host it on a different server. The practical use is that one could have a supplementary website, like an e-learning portal for college students as well as the main school website. If you are using subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a particular site, not mentioning that it'll be more safe to have the sites separate from one another.