A subdomain is a web address, that is under a main, or a root domain name. An illustration is something.domain.com, in which 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 a part of the whole web address, a subdomain can be fully independent and even hosted by a separate service provider. Using a subdomain can be extremely useful if you want to have different sites that are connected in some way. As an example, you can have an on-line store under the main domain and a forum under a subdomain so as to offer means for your clients to ask questions and / or share opinions about your products. The main advantage of using a subdomain rather than a subfolder is that the web sites will be separated, so you're able to update one of them or perform maintenance without affecting the other.