A subdomain is a website url, that is under a main, or a root domain. An example is something.domain.com, in which domain.com is the main domain address, while "something" is the subdomain below it. Every subdomain can have its own website and records i.e. apart from using the domain name as a part of the whole web address, a subdomain can be entirely independent and also hosted by a different company. Using a subdomain can be quite beneficial if you'd like to have different web sites that are connected in some way. For example, you could have a web-based store under the main domain and a forum under a subdomain in order to provide means for your clients to ask questions and / or share thoughts about your products. The advantage of 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.