A subdomain is a website url, which is under a main, or a root domain name. An example is something.domain.com, where domain.com is the main domain address, while "something" is the subdomain below it. Every single subdomain can have its own website and records i.e. apart from using the domain name as part of the whole web address, a subdomain can be totally independent and even hosted by a different service provider. Employing a subdomain could be very handy if you want to have different web sites which are related in some way. For instance, you may have a web-based store under the main domain as well as a forum under a subdomain so as to provide means for your customers to ask questions or share views about your products. The benefit of using a subdomain instead of a subfolder is that the websites are going to be separated, so you're able to update one of them or perform maintenance without affecting the other.