A subdomain is a web address, which is under a main, or a root domain name. An example is something.domain.com, in which domain.com is the main domain address, while "something" is the subdomain below it. Every single subdomain can have its own website and records i.e. apart from using the domain name as a 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 quite practical if you want to have different sites that are connected in some way. For 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 clients to ask questions and / or share experiences concerning your products. The advantage of employing a subdomain instead of a subfolder is that the sites will be separated, so you can update one of them or perform maintenance not affecting the other.