In IIS 7 or above, application pools can run in 2 managed pipeline modes. 1) Integrated, 2) Classic.
1. Integrated Mode
+ By default, IIS application pools run in this mode
+ IIS will use the integrated request processing pipeline to process all IIS requests, including ASP.NET requests
2. Classic Mode
+ IIS will process the requests same as if the application running in IIS 6.0
+ IIS will route ASP.NET requests to Aspnet_isapi.dll ISAPI extension