A subdomain is a website url, that's under a main, or a root domain name. An example is something.domain.com, in which domain.com is the main domain, while "something" is the subdomain below it. Each subdomain can have its own site and records i.e. apart from employing the domain name as part of the whole web address, a subdomain can be totally independent and even hosted by a separate provider. Employing a subdomain could be very handy if you'd like to have different web sites which are related in some way. For instance, you may have an on-line 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 / or share views about your products. The main advantage of employing a subdomain rather than a subfolder is that the websites are going to be separated, so you can update one of them or perform maintenance not affecting the other.