A subdomain is a website url, that's under a main, or a root domain name. An example is something.domain.com, in which domain.com is the main domain address, while "something" is the subdomain below it. Each subdomain can have its own website and records i.e. apart from employing the domain name as part of the whole web address, a subdomain can be completely independent and even hosted by a different service provider. Employing a subdomain could be very handy if you'd like to have different web sites that are connected in some way. For example, you could have an online store under the main domain along with a forum under a subdomain so as to provide means for your clients to ask questions or share opinions concerning your products. The best thing about employing a subdomain as opposed to a subfolder is that the sites are going to be separated, so you're able to update one of them or perform maintenance without affecting the other.