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. Every subdomain can have its own site and records i.e. apart from employing the domain name as a part of the whole web address, a subdomain can be completely independent and even hosted by a different provider. Employing a subdomain can be very useful if you want to have different websites which are associated in some way. For example, you can have a web-based store under the main domain and a forum under a subdomain so as to offer means for your customers to ask questions and share thoughts concerning your products. The advantage of using a subdomain as opposed to a subfolder is that the sites are going to be separated, so you can update one of them or perform maintenance not affecting the other.