A subdomain is the part of the web address that is before a domain name and you have quite possibly seen a lot of subdomains while browsing world wide web. For example, many websites such as Wikipedia have versions a number of 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 website and its own records, so you're able to even host it on a separate server. The practical use is that you could have a supplementary site, such as an e-learning portal for students on top of the primary school website. If you are using subdomains as opposed to subfolders, it'll 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 each other.