A subdomain is a website address, that is under a main, or a root domain. One 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. apart from using the domain name as part of the whole web address, a subdomain can be totally independent and also hosted by a different service provider. Employing a subdomain can be extremely useful if you'd like to have different web sites which are associated in some way. For example, you may have an online store under the main domain along with a forum under a subdomain in order to offer means for your clients to ask questions and / or share experiences concerning your products. The advantage of employing a subdomain as opposed to a subfolder is that the web sites will be separated, so you're able to update one of them or perform maintenance without affecting the other.