A subdomain is a website url, which is under a main, or a root domain name. An illustration 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 completely independent and even hosted by a different provider. Using a subdomain can be quite handy if you want to have different websites that are associated in some way. For example, you could have an on-line store under the main domain as well as a forum under a subdomain in order to supply means for your customers to ask questions and share thoughts concerning your products. The advantage of using a subdomain rather than a subfolder is that the sites will be separated, so you are able to update one of them or perform maintenance without affecting the other.