A subdomain is a website address, that's 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. Every subdomain can have its own website and records i.e. apart from using the domain name as part of the whole web address, a subdomain can be entirely independent and also hosted by a separate service provider. Employing a subdomain could be very beneficial if you want to have different web sites which are associated in some way. For instance, you can have a web-based store under the main domain along with a forum under a subdomain so as to offer means for your clients to ask questions and / or share views about your products. The advantage of using a subdomain rather than a subfolder is that the websites are going to be separated, so you can update one of them or perform maintenance not affecting the other.