From Facebook Developer Wiki

The Create Application API makes it easy for developers to enable their users to integrate social context on their blogs and websites.

Using this API generates an application name, an API key, an application secret, and a new creator_app setting (which identifies the parent application). From there the user can use the Developer application to configure the application's settings, or the parent application can manage them through the permissions API. This API will also work for applications developed on Facebook.com.

There are two ways to call the Create Application API:

• FB.Connect.createApplication -- the JavaScript Client Library method.
• Facebook.createApplication -- the FBJS method.

See Also

For more information about enabling others to create widgets and applications, see Facebook Connect Fourth Party Code.