A subdomain is a website address, that's under a main, or a root domain name. An example is something.domain.com, where domain.com is the main domain name, while "something" is the subdomain below it. Every single subdomain can have its own site and records i.e. aside from using the domain name as part of the whole web address, a subdomain can be totally independent and even hosted by a separate service provider. Employing a subdomain could be very beneficial if you wish to have different websites that are associated in some way. For example, you can have a web-based store under the main domain as well as a forum under a subdomain in order to offer means for your clients to ask questions and / or share opinions concerning your products. The best thing about employing a subdomain rather than a subfolder is that the sites are going to be separated, so that you can update one of them or perform maintenance without affecting the other.