A subdomain is a website address, which is under a main, or a root domain. An example is something.domain.com, in which 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 a part of the whole web address, a subdomain can be entirely independent and also hosted by a separate company. Using a subdomain could be very beneficial if you want to have different websites that are connected in some way. As an example, you could have an online 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 views about your products. The main advantage of employing a subdomain rather than a subfolder is that the websites will be separated, so you're able to update one of them or perform maintenance without affecting the other.