A subdomain is a website url, that is under a main, or a root domain. 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 website and records i.e. apart from using the domain name as a part of the whole web address, a subdomain can be totally independent and even hosted by a separate company. Using a subdomain could be very beneficial if you want to have different sites which are associated in some way. As an example, you can have an online store under the main domain and a forum under a subdomain in order to supply means for your clients to ask questions and share experiences regarding your products. The best thing about using a subdomain instead of a subfolder is that the web sites are going to be separated, so you are able to update one of them or perform maintenance without affecting the other.