A subdomain is a website url, which is under a main, or a root domain name. An example is something.domain.com, where domain.com is the main domain address, while "something" is the subdomain below it. Every subdomain can have its own site and records i.e. aside from employing the domain name as part of the whole web address, a subdomain can be entirely independent and even hosted by a different service provider. Using a subdomain can be quite practical if you want to have different web sites that are associated in some way. As an example, you could 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 experiences concerning your products. The best thing about using a subdomain rather than a subfolder is that the web sites are going to be separated, so you can update one of them or perform maintenance not affecting the other.