A subdomain is a web address, which is under a main, or a root domain. An example is something.domain.com, in which domain.com is the main domain name, while "something" is the subdomain below it. Each subdomain can have its own site and records i.e. aside from using the domain name as a part of the whole web address, a subdomain can be totally independent and even hosted by a different provider. Using a subdomain could be very useful if you'd like to have different web sites that are connected in some way. As an example, you may have an online store under the main domain as well as a forum under a subdomain in order to offer means for your clients to ask questions and / or share views regarding your products. The benefit of using a subdomain as opposed to a subfolder is that the web sites are going to be separated, so you are able to update one of them or perform maintenance without affecting the other.