A subdomain is a web address, that is 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 website and records i.e. apart from using the domain name as part of the whole web address, a subdomain can be totally independent and also hosted by a separate company. Employing a subdomain can be extremely beneficial if you want to have different sites which are connected in some way. As an example, you may have a web-based store under the main domain as well as a forum under a subdomain in order to provide means for your customers to ask questions and share views about your products. The best thing about employing a subdomain rather than a subfolder is that the web sites are going to be separated, so that you can update one of them or perform maintenance without affecting the other.