A subdomain is a part of a web address that's under the main domain, 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 could have its own web site and records and can even be hosted through a different company if you want to use a specific feature that isn't 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 clients can buy your products. In addition, you can have a forum where they can comment on the products and by using subdomains as an alternative to subfolders you'll avoid any risk of all websites going down if you perform maintenance, or update one of the website scripts. Keeping the sites separated is more secure in case of a script security breach.