A subdomain is part of a web address that is under the main domain, such as name.example.com. Actually, even in www.example.com the "www" part is a subdomain given that the fully qualified domain name is only "example.com". Each subdomain could have its own website and records and can even be hosted with a different provider if you need to use a specific feature that is not provided by your current provider. A good example for using a subdomain is if you have a company website and an online store under a subdomain where customers can acquire your products. You can also have a forum where they can talk about the products and by using subdomains as opposed to subfolders you will 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 the event of a script security breach.