A subdomain is 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 since the fully qualified domain name is only "example.com". Each subdomain can have its own site and records and can also be hosted using a different provider if you need to use a certain feature which isn't provided by your current service provider. An example for using a subdomain is if you have a business website along with an online store under a subdomain where customers can purchase your products. You can even have a forum in which they can comment on the products and by using subdomains instead of subfolders you are going to avoid any chance of all sites going down when you perform maintenance, or update one of the website scripts. Keeping the websites separated is more secure in case there is a script security breach.