A subdomain is a web address, that's 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 site and records i.e. apart from using 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 useful if you wish to have different sites that are associated in some way. For example, you could have an on-line store under the main domain plus a forum under a subdomain in order to provide means for your clients to ask questions and / or share experiences about your products. The main advantage of using a subdomain as opposed to a subfolder is that the web sites are going to be separated, so you're able to update one of them or perform maintenance without affecting the other.