A subdomain is a web address, which is under a main, or a root domain. One example is something.domain.com, where domain.com is the main domain, while "something" is the subdomain below it. Each 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 fully independent and also hosted by a different provider. Using a subdomain could be very practical if you wish to have different web sites which are connected in some way. As an example, you could have an online store under the main domain along with a forum under a subdomain in order to supply means for your customers to ask questions and / or share experiences concerning your products. The advantage of using a subdomain rather than a subfolder is that the web sites will be separated, so that you can update one of them or perform maintenance not affecting the other.