A subdomain is a web 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. Every subdomain can have its own site and records i.e. aside from using the domain name as part of the whole web address, a subdomain can be completely independent and also hosted by a different company. Using a subdomain can be very beneficial if you want to have different sites which are connected in some way. For example, you could have an online store under the main domain and a forum under a subdomain in order to supply means for your customers to ask questions and / or share thoughts about your products. The advantage of using a subdomain rather than a subfolder is that the websites are going to be separated, so you're able to update one of them or perform maintenance not affecting the other.