A subdomain is a website url, which is under a main, or a root domain name. One example is something.domain.com, where domain.com is the main domain name, 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 a part of the whole web address, a subdomain can be fully independent and even hosted by a different provider. Using a subdomain could be very useful if you wish to have different websites that are related in some way. For instance, you could have a web-based store under the main domain plus a forum under a subdomain in order to provide means for your customers to ask questions and / or share opinions about your products. The benefit of employing a subdomain rather than 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.