A subdomain is a website address, that is under a main, or a root domain name. An example is something.domain.com, where domain.com is the main domain name, while "something" is the subdomain below it. Every subdomain can have its own website and records i.e. aside from using the domain name as a part of the whole web address, a subdomain can be fully independent and also hosted by a different service provider. Using a subdomain could be very beneficial if you'd like to have different sites that are associated in some way. For example, you can have an on-line store under the main domain and a forum under a subdomain in order to provide means for your customers to ask questions and share thoughts about your products. The main advantage of employing a subdomain rather than a subfolder is that the sites will be separated, so you can update one of them or perform maintenance not affecting the other.