A subdomain is a website url, that's under a main, or a root domain name. One example is something.domain.com, in which domain.com is the main domain, while "something" is the subdomain below it. Every subdomain can have its own site and records i.e. apart from employing the domain name as a part of the whole web address, a subdomain can be entirely independent and also hosted by a separate provider. Employing a subdomain can be extremely handy if you want to have different web sites which are associated in some way. As an example, you could have an online store under the main domain plus a forum under a subdomain in order to offer means for your customers to ask questions and / or share thoughts about your products. The best thing about using a subdomain as opposed to a subfolder is that the sites will be separated, so you are able to update one of them or perform maintenance without affecting the other.