A subdomain is a website address, 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 using the domain name as part of the whole web address, a subdomain can be entirely independent and even hosted by a separate provider. Using a subdomain can be extremely beneficial if you want to have different websites that are associated in some way. For example, you may have an online store under the main domain plus a forum under a subdomain in order to supply means for your customers to ask questions and share thoughts about your products. The benefit of employing a subdomain instead of a subfolder is that the websites are going to be separated, so that you can update one of them or perform maintenance without affecting the other.