A subdomain is a website url, that is under a main, or a root domain. An example is something.domain.com, in which domain.com is the main domain, 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 entirely independent and even hosted by a different service provider. Using a subdomain can be extremely practical if you'd like to have different websites which are related in some way. As an example, you can have an online store under the main domain plus a forum under a subdomain in order to provide means for your customers to ask questions and share experiences regarding your products. The benefit of employing a subdomain instead of a subfolder is that the sites are going to be separated, so you are able to update one of them or perform maintenance without affecting the other.