A subdomain is a website address, that's under a main, or a root domain. An 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 site and records i.e. apart from employing the domain name as part of the whole web address, a subdomain can be completely independent and also hosted by a separate company. Using a subdomain can be very practical if you'd like to have different sites that are related in some way. As an example, you may have an online store under the main domain plus a forum under a subdomain in order to provide means for your customers to ask questions and / or share views about your products. The advantage of using a subdomain instead of a subfolder is that the web sites will be separated, so you're able to update one of them or perform maintenance not affecting the other.