A subdomain is a web address, that is under a main, or a root domain. An 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 website and records i.e. apart from using the domain name as a part of the whole web address, a subdomain can be completely independent and even hosted by a separate provider. Using a subdomain can be quite beneficial if you wish to have different websites that are associated in some way. For example, you may have an online store under the main domain as well as a forum under a subdomain in order to supply means for your customers to ask questions and / or share views regarding your products. The advantage of using 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 not affecting the other.