A subdomain is a website url, that's 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. apart from employing the domain name as part of the whole web address, a subdomain can be fully independent and even hosted by a different company. Employing a subdomain can be quite handy if you wish to have different web sites that are associated in some way. For instance, you can have an online store under the main domain along with a forum under a subdomain so as to supply means for your customers to ask questions and / or share experiences regarding your products. The best thing about employing a subdomain instead of a subfolder is that the web sites are going to be separated, so you are able to update one of them or perform maintenance not affecting the other.