The NavigationService class is used in Windows Phone 8 App to navigate from one page to another but note that if you want to launch the external URL in Internet Explorer from the App, you cannot use the NavigationService.
You will receive the following error, if you are attempting to open the external URL using Navigation Service.
{System.ArgumentException: Navigation is only supported to relative URIs that are fragments, or begin with ‘/’, or which contain ‘;component/’.
Parameter name: uri at System.Windows.Navigation.NavigationService.Navigate(Uri source)}
In order to open the external URL from the application, the best option is to use the WebBrowserTask or WebBrowserControl within the application.
