A subdomain is a web address, that is under a main, or a root domain name. An illustration 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 website and records i.e. apart from employing the domain name as part of the whole web address, a subdomain can be completely independent and also hosted by a different provider. Employing a subdomain can be extremely handy if you wish to have different web sites that are related in some way. For example, you can have an online store under the main domain plus a forum under a subdomain in order to provide means for your clients to ask questions and share experiences concerning your products. The advantage of employing a subdomain rather than a subfolder is that the sites are going to be separated, so that you can update one of them or perform maintenance without affecting the other.