A subdomain is a web 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. Each and every 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. Using a subdomain can be very beneficial if you'd like to have different websites that are associated in some way. For instance, you could have a web-based store under the main domain along with a forum under a subdomain so as to offer means for your clients to ask questions or share thoughts concerning your products. The main advantage of using 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.