A subdomain is a web address, 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 using the domain name as a part of the whole web address, a subdomain can be completely independent and also hosted by a separate service provider. Employing a subdomain could be very practical if you'd like to have different web sites which are associated in some way. For instance, you could have an on-line 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 share thoughts concerning your products. The main advantage of using a subdomain as opposed to 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.