A subdomain is a part of a web address which is under the main domain name, such as name.example.com. Technically, even in www.example.com the "www" part is a subdomain given that the fully qualified domain name is only "example.com". Every single subdomain can have its own web site and records and can also be hosted through a different provider if you want to use some feature which is not provided by your current provider. An example for using a subdomain is if you have a business website and an online store under a subdomain where customers can purchase your products. Additionally, you can have a forum where they can talk about the products and by employing subdomains rather than subfolders you are going to avoid any probability of all sites going down if you perform maintenance, or update one of the site scripts. Keeping the websites separated is also more secure in the event of a script security breach.