A subdomain is a web address, that's under a main, or a root domain. An example is something.domain.com, in which domain.com is the main domain, 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 totally independent and also hosted by a separate service provider. Using a subdomain can be extremely practical if you want to have different sites that are related in some way. As an example, you may have an on-line store under the main domain along with a forum under a subdomain in order to provide means for your customers to ask questions and share views about your products. The main advantage of employing a subdomain rather than a subfolder is that the sites will be separated, so that you can update one of them or perform maintenance not affecting the other.