http://www.xefteri.com/articles/show.cfm?id=18

Control (Server side)  => Html tag + Javascript (Client side)
1, if a button is clicked on the client side,  then the button fire the event (__doPostBack) implmented in JS. And then pass the method with parameter needed.
2, the the __doPostBack method then passes the information about the client to the server,  then the server identifies which control and which event.
3, the server then deals with the event