A subdomain is a web address, that's under a main, or a root domain name. One example is something.domain.com, where domain.com is the main domain address, while "something" is the subdomain below it. Each subdomain can have its own website and records i.e. aside from employing the domain name as part of the whole web address, a subdomain can be totally independent and also hosted by a different company. Using a subdomain can be extremely handy if you want to have different sites which are associated in some way. For example, you can have a web-based store under the main domain plus a forum under a subdomain in order to offer means for your clients to ask questions or share views regarding your products. The advantage of using a subdomain rather than 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.