HTTP handlers and HTTP modules:

HTTP handlers :

ASP.NET uses HTTP handlers to process all requests.

Every request flows through a number of HTTP modules, which cover various areas of the application (i.e. authentication and session intofmation). After passing through each module, the request is assigned to a single HTTP handler, which determines how the system will respond to the request. Upon completion of the request handler, the response flows back through the HTTP modules to the user.

All requests to IIS are handled through Internet Server Application Programming Interface (ISAPI) extensions.

An ASP.NET HTTP handler is the process (frequently referred to as the "endpoint") that runs in response to a request made to an ASP.NET Web application. The most common handler is an ASP.NET page handler that processes .aspx files. When users request an .aspx file, the request is processed by the page through the page handler. You can create your own HTTP handlers that render custom output to the browser.

HTTP modules:

An HTTP module is an assembly that is called on every request that is made to your application. HTTP modules are called as part of the ASP.NET request pipeline and have access to life-cycle events throughout the request. HTTP modules let you examine incoming and outgoing requests and take action based on the request.