A subdomain is a website address, that's under a main, or a root domain. An example is something.domain.com, where domain.com is the main domain, while "something" is the subdomain below it. Every subdomain can have its own website and records i.e. aside from employing the domain name as part of the whole web address, a subdomain can be totally independent and even hosted by a separate service provider. Using a subdomain can be quite useful if you want to have different web sites that are related in some way. For example, you may have an on-line store under the main domain as well as a forum under a subdomain so as to offer means for your clients to ask questions and / or share thoughts regarding your products. The benefit of employing a subdomain rather than a subfolder is that the sites are going to be separated, so that you can update one of them or perform maintenance not affecting the other.