A subdomain is a part of a web address which is under the main domain name, such as name.example.com. Actually, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is just "example.com". Every subdomain can have its own site and records and can even be hosted using a different company if you need to use a feature that's not offered by your current service provider. One example for using a subdomain is if you have a company site along with an online store under a subdomain where customers can purchase your products. Additionally, you can have a forum where they can discuss the products and by using subdomains as an alternative to subfolders you are going to avoid any risk of all websites going down if you perform maintenance, or update one of the site scripts. Keeping your websites separated is also more secure in the event of a script security breach.