An external application is an application not managed by the server you control. This means that you cannot easily setup single sign on with that application. WebCenter provides a technique to automatically post the credentials from your users to these applications so they have the idea that it is SSO-enabled.
This way you can easily integrate other web applications into your portal and your users don't need to navigate away from the portal.
In this recipe, we will register Google groups as an external application so we can integrate the Google groups functionality in our portal.
For this recipe, you need a WebCenter Portal application.