A subdomain is a website url, that's under a main, or a root domain. An example is something.domain.com, in which domain.com is the main domain name, while "something" is the subdomain below it. Every single subdomain can have its own website and records i.e. aside 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. Using a subdomain can be extremely handy 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 in order to provide means for your clients to ask questions or share views regarding your products. The best thing about using a subdomain rather than a subfolder is that the sites will be separated, so you are able to update one of them or perform maintenance not affecting the other.