A subdomain is a website address, which is 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. Every single subdomain can have its own site and records i.e. apart from using the domain name as a part of the whole web address, a subdomain can be totally 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 associated in some way. For instance, you may have an on-line store under the main domain plus a forum under a subdomain in order to offer means for your clients to ask questions or share opinions regarding your products. The benefit of using a subdomain rather than a subfolder is that the websites are going to be separated, so you're able to update one of them or perform maintenance not affecting the other.