Windows Process Activation Service (WAS) was first introduced in Windows Vista and Windows Server 2008 as an application environment that extends IIS 7 and later to provide first-class support for hosting WCF and WF services.
Built on top of IIS, WAS provides additional capabilities such as:
Message-based activation, which ensures that processes and applications are only activated when there are requests arriving at the service
Support for both HTTP and non-HTTP protocols such as NetTcp, NetMSMQ, and NetPipes
Support for Auto-Start, which allows for expensive initialization work for a service to take place when the application is started as opposed to waiting for the first message to arrive
Intelligent application recycling to provide automatic health management of problematic applications by recycling the worker process based on number of requests, memory thresholds, or at a scheduled time
Windows Server AppFabric builds on and extends these capabilities...