A subdomain is a web address, that's under a main, or a root domain. An example is something.domain.com, where domain.com is the main domain name, while "something" is the subdomain below it. Every single subdomain can have its own site and records i.e. aside from using the domain name as part of the whole web address, a subdomain can be completely independent and even hosted by a separate service provider. Employing a subdomain could be very handy if you wish to have different web sites that are associated in some way. As an example, you could have a web-based store under the main domain plus a forum under a subdomain in order to provide means for your clients to ask questions and share views regarding your products. The benefit of employing a subdomain rather than a subfolder is that the websites will be separated, so that you can update one of them or perform maintenance without affecting the other.