update panal concept help

every time i will use update panal to catch whole page without using the trigger? maybe this not a real method. can any tell me how to use trigger.

for example

<asp:UpdatePanel ID="UpdatePanel1" runat="server"><ContentTemplate>

            /*this is what i use every time when i press i will not postback but it will make whole page shake a while. how to use trigger only target the thing? for example i want press a button the thing on div chg, how should i do? */
<div id="eg1"><div><asp:Button ID="Button" runat="server" Text="Button" /></ContentTemplate></asp:UpdatePanel>