A subdomain is a website url, that is under a main, or a root domain name. One example is something.domain.com, where domain.com is the main domain, while "something" is the subdomain below it. Every subdomain can have its own website and records i.e. apart from employing the domain name as part of the whole web address, a subdomain can be entirely independent and even hosted by a different company. Using a subdomain can be quite useful if you'd like to have different sites that are associated in some way. As an example, you can have a web-based store under the main domain and a forum under a subdomain in order to offer means for your clients to ask questions and / or share opinions concerning your products. The advantage of using a subdomain as opposed to a subfolder is that the web sites are going to be separated, so you are able to update one of them or perform maintenance not affecting the other.