A subdomain is a website address, which is under a main, or a root domain. An illustration is something.domain.com, where domain.com is the main domain name, while "something" is the subdomain below it. Every 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 even hosted by a different provider. Using a subdomain can be quite beneficial if you wish to have different websites that are associated in some way. As an example, you could have an on-line store under the main domain plus a forum under a subdomain so as to provide means for your clients to ask questions or share views about your products. The benefit of employing a subdomain as opposed to a subfolder is that the websites will be separated, so you're able to update one of them or perform maintenance not affecting the other.