A subdomain is a website url, which is under a main, or a root domain. One example is something.domain.com, in which domain.com is the main domain, while "something" is the subdomain below it. Every single subdomain can have its own site and records i.e. aside from using the domain name as a part of the whole web address, a subdomain can be entirely independent and also hosted by a different service provider. Using a subdomain can be extremely handy if you want to have different sites which are associated in some way. For example, you could have an online store under the main domain as well as a forum under a subdomain so as to supply means for your clients to ask questions or share experiences regarding your products. The main advantage of using a subdomain as opposed to a subfolder is that the sites are going to be separated, so you're able to update one of them or perform maintenance without affecting the other.