A subdomain is a website url, which is under a main, or a root domain name. An example is something.domain.com, in which domain.com is the main domain address, while "something" is the subdomain below it. Each 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 also hosted by a different service provider. Using a subdomain could be very beneficial if you want to have different websites that are associated in some way. For example, you could have a web-based store under the main domain plus a forum under a subdomain so as to provide means for your customers to ask questions or share experiences about your products. The benefit of employing a subdomain as opposed to a subfolder is that the websites are going to be separated, so you're able to update one of them or perform maintenance without affecting the other.