A subdomain is part of a web address that is under the main domain, such as name.example.com. Actually, even in www.example.com the "www" part is a subdomain given that the fully qualified domain name is just "example.com". Every single subdomain could have its own site and records and can also be hosted through a different provider if you need to use a specific feature that isn't offered by your current service provider. An illustration for using a subdomain is if you have a business site as well as an online store under a subdomain where clients can purchase your products. In addition, you can have a forum in which they can comment on the products and by using subdomains as an alternative to subfolders you'll avoid any chance of all sites going down if you perform maintenance, or update one of the site scripts. Keeping your websites separated is less risky in case of a script security breach.