A subdomain is a website url, that's under a main, or a root domain. An example is something.domain.com, where domain.com is the main domain address, while "something" is the subdomain below it. Each and every subdomain can have its own website and records i.e. apart from employing the domain name as part of the whole web address, a subdomain can be entirely independent and even hosted by a different provider. Using a subdomain can be quite beneficial if you wish to have different sites that are associated in some way. For example, you may have an on-line store under the main domain as well as a forum under a subdomain in order to offer means for your customers to ask questions and share thoughts about your products. The best thing about using a subdomain instead of a subfolder is that the websites are going to be separated, so you can update one of them or perform maintenance without affecting the other.