A subdomain is a part of a web address that is under the main domain name, for instance 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 just "example.com". Every subdomain could have its own web site and records and can also be hosted with a different provider if you want to use a feature that isn't provided by your current service provider. An example for using a subdomain is if you have a company website along with an online store under a subdomain where customers can acquire your products. Additionally, you can have a forum where they can talk about the products and by employing subdomains instead of subfolders you are going to avoid any risk of all websites going down if you perform maintenance, or update one of the site scripts. Keeping the sites separated is also more secure in the case of a script security breach.