A subdomain is part of a web address which is under the main domain name, such as 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 only "example.com". Every subdomain can have its own site and records and can also be hosted with a different provider if you need to use a certain feature which is not offered by your current service provider. A good example for using a subdomain is if you have a business website along with an online store under a subdomain where customers can buy your products. You can also have a forum where they can talk about the products and by using subdomains as opposed to subfolders you'll avoid any risk of all sites going down when you perform maintenance, or update one of the website scripts. Keeping your websites separated is more secure in case of a script security breach.