What is AJAX?
Fig 1: AJAX Extensions Toolbox
Fig 2: How to use AJAX ScriptManger
Fig 3: How to use AJAX UpdatePanel
The final step is to write code for server side operations. Write respective server side code inside the particular controllers' (Eg: button, menu...) event handling methods same as you do in normally applications. But here different is this server side code will execute on server and send results to corresponding controller but no page refresh.
Fig 4: How to write server side coding in AJAX