A subdomain is a website address, that's under a main, or a root domain. An example is something.domain.com, in which domain.com is the main domain, while "something" is the subdomain below it. Every single 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 also hosted by a separate company. Using a subdomain can be very useful if you wish to have different websites which are related in some way. As an example, you may have an on-line store under the main domain as well as a forum under a subdomain so as to provide means for your clients to ask questions or share thoughts about your products. The main advantage of employing a subdomain instead of a subfolder is that the websites will be separated, so that you can update one of them or perform maintenance without affecting the other.