A subdomain is part of a web address that's under the main domain, 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 site and records and can even be hosted through a different company if you wish to use a feature which isn't provided by your current provider. One example for using a subdomain is if you have a company site and an online store under a subdomain where clients can acquire your products. You can also have a forum where they can talk about the products and by using subdomains instead of subfolders you are going to avoid any risk of all sites going down when you perform maintenance, or update one of the website scripts. Keeping your sites separated is more secure in case there is a script security breach.