A subdomain is a website url, that's under a main, or a root domain name. An example is something.domain.com, where domain.com is the main domain name, 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 provider. Employing a subdomain can be extremely handy if you'd like to have different websites which are associated in some way. For instance, you can have an online 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 experiences regarding your products. The benefit of using a subdomain instead of a subfolder is that the websites are going to be separated, so you are able to update one of them or perform maintenance without affecting the other.