A subdomain is a website address, that is under a main, or a root domain name. One 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 site and records i.e. aside from using the domain name as a part of the whole web address, a subdomain can be totally independent and even hosted by a separate provider. Using a subdomain can be quite handy if you want to have different websites that are associated in some way. For instance, you may have an on-line store under the main domain along with a forum under a subdomain so as to offer means for your clients to ask questions and share experiences about your products. The best thing about employing a subdomain rather than a subfolder is that the sites will be separated, so you can update one of them or perform maintenance without affecting the other.