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 name, 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 totally independent and also hosted by a separate service provider. Employing a subdomain can be extremely handy if you want to have different sites that are related in some way. For example, you could have an online store under the main domain and a forum under a subdomain so as to provide means for your clients to ask questions and / or share opinions about your products. The main advantage of using a subdomain instead of a subfolder is that the sites are going to be separated, so you are able to update one of them or perform maintenance not affecting the other.