A subdomain is a web address, which is under a main, or a root domain name. One example is something.domain.com, where domain.com is the main domain, while "something" is the subdomain below it. Each subdomain can have its own site and records i.e. aside from using the domain name as a part of the whole web address, a subdomain can be totally independent and also hosted by a different service provider. Employing a subdomain can be very handy if you wish to have different sites which are connected in some way. For example, you may have a web-based store under the main domain plus a forum under a subdomain in order to provide means for your clients to ask questions and / or share views about your products. The advantage of using a subdomain rather than a subfolder is that the web sites will be separated, so you are able to update one of them or perform maintenance without affecting the other.