A subdomain is a web address, that's under a main, or a root domain. One example is something.domain.com, where domain.com is the main domain name, while "something" is the subdomain below it. Every subdomain can have its own site and records i.e. aside from employing the domain name as a part of the whole web address, a subdomain can be entirely independent and even hosted by a separate company. Using a subdomain could be very beneficial if you want to have different web sites that are related in some way. As an example, you could have a web-based store under the main domain as well as a forum under a subdomain so as to offer means for your clients to ask questions and / or share experiences concerning your products. The advantage of employing a subdomain instead of a subfolder is that the sites are going to be separated, so you are able to update one of them or perform maintenance without affecting the other.