A subdomain is a website address, which is under a main, or a root domain. One example is something.domain.com, where domain.com is the main domain, 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 part of the whole web address, a subdomain can be totally independent and even hosted by a different company. Using a subdomain can be quite beneficial if you want to have different web sites that are related in some way. For instance, you may have an online store under the main domain and a forum under a subdomain so as to supply means for your clients to ask questions and share thoughts concerning your products. The main advantage of employing a subdomain as opposed to a subfolder is that the sites will be separated, so that you can update one of them or perform maintenance without affecting the other.