A subdomain is a website address, that's under a main, or a root domain name. An example is something.domain.com, where domain.com is the main domain address, while "something" is the subdomain below it. Each subdomain can have its own website and records i.e. aside from using the domain name as part of the whole web address, a subdomain can be entirely independent and also hosted by a separate service provider. Using a subdomain could be very useful if you wish to have different sites which are connected in some way. For instance, you can have an online store under the main domain and a forum under a subdomain in order to offer means for your clients to ask questions and share thoughts regarding your products. The best thing about using a subdomain rather than a subfolder is that the sites will be separated, so that you can update one of them or perform maintenance without affecting the other.