A subdomain is a website address, that is under a main, or a root domain name. One example is something.domain.com, in which domain.com is the main domain name, while "something" is the subdomain below it. Each subdomain can have its own site and records i.e. aside from employing the domain name as a part of the whole web address, a subdomain can be totally independent and even hosted by a different service provider. Employing a subdomain can be extremely handy if you wish to have different websites which are connected in some way. For example, you could have an online store under the main domain as well as a forum under a subdomain in order to provide means for your clients to ask questions or share experiences concerning your products. The main advantage of using a subdomain instead of a subfolder is that the web sites will be separated, so you can update one of them or perform maintenance without affecting the other.