A subdomain is a web address, which is under a main, or a root domain. An illustration is something.domain.com, in which domain.com is the main domain, while "something" is the subdomain below it. Each and every 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 fully independent and also hosted by a separate company. Using a subdomain could be very beneficial if you wish to have different web sites that are related in some way. For instance, you could have an on-line store under the main domain along with a forum under a subdomain in order to offer means for your clients to ask questions and share experiences concerning your products. The benefit of using a subdomain rather than 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.