A subdomain is the part of the web address that's before a domain and you have most likely seen a lot of subdomains while surfing around the Web. As an illustration, many sites like Wikipedia have versions in various 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 website and its own records, so you can even host it on another server. The practical use is that one could have a supplementary site, such as an e-learning portal for college students in addition to the primary school site. If you use subdomains as opposed to subfolders, it's going to 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.