When the update panel is refreshed then which event of the update panel is fired on the server side?
The UpdatePanel control from the Ajax Control Toolkit is more of a container control. It is a container that frees you from writing the plumbing for asynchronous ajax calls.
What that means is that this you will put your regular controls like buttons / dropdownlists etc that you want within this UpdatePanel Control.
If, for example, you want the button click to fire a server side event and do some processing, you will associate the button click event for the button just like you normally would in asp.net.
Because of the button being within the UpdatePanel (and provided the ChildrenAsTriggers / UpdateMode properties are set correctly) it is the buttons "OnClick" event that will be fired on the server side when you click the button.