A subdomain is a web address, that's under a main, or a root domain name. An example is something.domain.com, in which domain.com is the main domain, while "something" is the subdomain below it. Each subdomain can have its own website and records i.e. aside from using the domain name as part of the whole web address, a subdomain can be totally independent and also hosted by a separate company. Using a subdomain can be extremely useful if you'd like to have different sites that are associated in some way. For instance, you can have an on-line store under the main domain along with a forum under a subdomain in order to supply means for your clients to ask questions and share opinions concerning your products. The best thing about using a subdomain as opposed to a subfolder is that the sites are going to be separated, so you're able to update one of them or perform maintenance not affecting the other.