A subdomain is a website address, which is under a main, or a root domain. One example is something.domain.com, where domain.com is the main domain, while "something" is the subdomain below it. Every subdomain can have its own site and records i.e. apart from employing 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 extremely handy if you want to have different websites that are connected in some way. As an example, you can have a web-based store under the main domain as well as a forum under a subdomain so as to provide means for your customers to ask questions and share thoughts about your products. The advantage of using a subdomain rather than a subfolder is that the web sites are going to be separated, so you're able to update one of them or perform maintenance not affecting the other.