Extend a Web application

If you want to expose the same content in a Web application to different types of users by using additional URLs or authentication methods, you can extend an existing Web application into a new zone. When you extend the Web application into a new zone, you create a separate Internet Information Services (IIS) Web site to serve the same content, but with a unique URL and authentication type.

An extended Web application can use up to five network zones (Default, Intranet, Internet, Custom, and Extranet). For example, if you want to extend a Web application so that customers can access content from the Internet, you select the Internet zone and choose to allow anonymous access and grant anonymous users read-only permissions. Customers can then access the same Web application as internal users, but through different URLs and authentication settings

http://technet.microsoft.com/en-us/library/cc261698(v=office.14).aspx