A subdomain is the part of the web address that's before a domain and you've almost certainly seen a lot of subdomains while browsing the Web. For instance, many websites like 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're able to even host it on a separate 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 are using subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a specific site, not mentioning that it will be more secure to have the sites separate from each other.