A subdomain is the part of the web address that is before a domain and you have quite possibly seen a lot of subdomains while surfing around world wide web. For instance, many sites like Wikipedia have versions in various 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're able to even host it on a separate server. The practical use is that one could have a supplementary site, like an e-learning portal for students aside from the main school site. If you are using subdomains as opposed to subfolders, it will 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.