A subdomain is a web address, that's under a main, or a root domain. An illustration is something.domain.com, in which domain.com is the main domain address, while "something" is the subdomain below it. Each 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 entirely independent and also hosted by a separate company. Employing a subdomain could be very beneficial if you want to have different web sites that are related in some way. For example, you may have a web-based store under the main domain along with a forum under a subdomain so as to provide means for your clients to ask questions and / or share views about your products. The main advantage of using a subdomain rather than a subfolder is that the web sites will be separated, so that you can update one of them or perform maintenance without affecting the other.