A subdomain is a website url, that's under a main, or a root domain name. An illustration is something.domain.com, where domain.com is the main domain, while "something" is the subdomain below it. Each and 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 fully independent and also hosted by a different company. Using a subdomain can be very beneficial if you'd like to have different web sites which are associated in some way. For example, you could have a web-based store under the main domain as well as a forum under a subdomain so as to provide means for your clients to ask questions and / or share opinions regarding your products. The benefit of employing a subdomain rather than a subfolder is that the websites will be separated, so you can update one of them or perform maintenance not affecting the other.