A subdomain is a part of a web address that is under the main domain name, for example name.example.com. Technically, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is just "example.com". Every single subdomain could have its own web site and records and can also be hosted using a different provider if you need to use a specific feature that is not offered by your current service provider. A good example for using a subdomain is if you have a business site along with an online store under a subdomain where clients can acquire your products. Additionally, you can have a forum in which they can comment on the products and by employing subdomains instead of subfolders you'll avoid any chance of all websites going down if you perform maintenance, or update one of the site scripts. Keeping your sites separated is less risky in the case of a script security breach.