What is the difference between Application_start and Session_start?

When 10 users (10 browsers) open a webpage, there are 10 sessions created and one application object created.

Application_Start will start before first page rendering after IIS starts.
Session_start runs on every new user(new browser window).

built-in Visual Studio Web server -Cassini

IIS would intercept requests and hand them off to specifc ISAPI DLLs to process the request

ASP(before ASP.NET) was actually handled by a single ISAPI DLL named ASP.DLL.

Earlier versions of ASP.NET relied on an ISAPI DLL named ASPNET_ISAPI.DLL

ASP.NET pipeline is now actually part of iis