what is Windows Process Activation Service ?

Answer:

The Windows Process Activation Service (WAS) provides process activation, resource management and health management services for message-activated applications. It manages application pool configuration and the creation and lifetime of worker processes for HTTP and other protocols (net.tcp,net.pipe,net.msmq)

Details : http://techprudent.com/the-windows-process-activation-service/
and
MSDN : http://technet.microsoft.com/en-us/library/cc735229%28WS.10%29.aspx