A subdomain is a web address, that's under a main, or a root domain name. An illustration is something.domain.com, where domain.com is the main domain name, while "something" is the subdomain below it. Each subdomain can have its own site and records i.e. apart from using the domain name as part of the whole web address, a subdomain can be entirely independent and also hosted by a different provider. Using a subdomain could be very beneficial if you want to have different websites which are related in some way. As an example, you could have a web-based store under the main domain along with a forum under a subdomain in order to offer means for your customers to ask questions and share thoughts about your products. The best thing about using a subdomain as opposed to a subfolder is that the web sites are going to be separated, so you're able to update one of them or perform maintenance without affecting the other.