A subdomain is a website address, that is under a main, or a root domain name. One example is something.domain.com, in which domain.com is the main domain, while "something" is the subdomain below it. Every subdomain can have its own site and records i.e. aside from using the domain name as part of the whole web address, a subdomain can be totally independent and also hosted by a different company. Using a subdomain can be quite practical if you want to have different web sites which are associated in some way. For instance, you can 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 and share opinions 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.