What is request-processing pipeline?
The request processing pipeline in IIS is the mechanism by which requests are processed beginning with a Request and ending with a Response.
Pictures speak louder than words, you should review the IIS7 architecture diagrams on this page:
Introduction to IIS 7 Architecture - HTTP Request Processing in IIS 7
IIS contains several components that perform important functions for the application and Web server roles in Windows Server® 2008 (IIS 7.0) and Windows Server 2008 R2 (IIS 7.5). Each component has responsibilities, such as listening for requests made to the server, managing processes, and reading configuration files. These components include protocol listeners, such as HTTP.sys, and services, such as World Wide Web Publishing Service (WWW service) and Windows Process Activation Service (WAS).