I think you are asking about the application server which comes with Windows Operating System i.e. IIS (Internet Information Service). Till IIS 6.0 its an WebServer but from IIS7.0 it has been transformed into Application Server. You can find this component in the Add/Remove Window components section of Control Panel. Coming to testing, you can perform various testings using this such as Installation Testing, Compatibility Testing, Functional Testing and Performance Testing.
Installation Testing: You can Install IIS from Control Panel and test weather all the required IIS components are installed successfully are not.
Compatibility Testing: You can Install IIS and test the compatbility with the different kind of applications
Functional Testing : IIS serves different functionalities mainly hosting websites. You can verify the different functionalities of IIS.
Perfomance Testing: As this is an application server we can test the no.of requests it can handle at a given time interval.
Likewise , you can perform wide variaty of testing around the Windows Application Server
Win App Server is not IIS. IIS is a Web server not APP Server.
In Windows Server® 2008, the new, expanded version of the Application Server role provides an integrated environment for deploying and running custom, server-based business applications. Typically, these business applications are developed within an organization or by an independent software vendor (ISV) for the organization.
For an overview of the improvements in Application Server, see the next section. For details about the improvements, see Application Server Role.
Overview of the improvements in Application Server
Application Server in Windows Server 2008 includes improvements in the following areas that simplify the process of installing and configuring the Application Server role in your environment so that you can deploy and run your organization's applications:
* A new, user-friendly Add Roles Wizard that helps you choose the services and features that are necessary to run your applications
* Application Server Foundation, the default installation of Application Server that includes the .NET Framework 3.0 features (If you have applications that are built with the .NET Framework 3.5, you can download and install the .NET Framework 3.5 onto the operating system.)
* Web Server: Application Server installs Internet Information Services (IIS) 7.0
* Windows Process Activation Service (WAS), which dynamically stops and starts applications based on messages that are received over the network
* Net.TCP Port Sharing, which makes it possible for multiple Windows Communication Foundation (WCF) applications to share a single TCP port for incoming communications