A subdomain is part of a web address that is under the main domain, such as name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain given that the fully qualified domain name is only "example.com". Every subdomain could have its own site and records and can also be hosted with a different provider if you need to use a feature that isn't provided by your current provider. A good example for using a subdomain is if you have a business website as well as an online store under a subdomain where clients can acquire your products. Additionally, you can have a forum where they can discuss the products and by using subdomains as opposed to subfolders you'll avoid any risk of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the sites separated is less risky in case of a script security breach.