This new property (*not* event), allows you to wire up clientside code to be executed before the server-side onclick is handled.  In the old days we had to do this manually.  For example, this is how you would hook up a simple confirmation dialog:
<script language="jscript" type="text/jscript"><!--  function deleteConfirmation(event)  {    if (!window.confirm("Are you sure?"))    {      window.event.returnValue = false;    }  }--></script><asp:LinkButton ID="deleteButton"                    runat="server"                    CausesValidation="false"                    CommandName="Delete"                   OnClick="deleteButton_Click"                   OnClientClick="deleteConfirmation()"                   Text="Delete"></asp:LinkButton>
This allows you to cancel the server-side event if the user says nope!  Cool!