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. Every single 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 provider. Using a subdomain can be very handy if you'd like to have different websites which are related in some way. For example, you can have a web-based store under the main domain plus a forum under a subdomain so as to provide means for your clients to ask questions and / or share opinions concerning your products. The main advantage of employing 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.