Default Button in ASP.net

hi..i have seen many of them dont know the usage of setting default button..

By reading this one you can get clear idea about uses of setting Default Button..
In an webpage when creatiing form like an application or any fill up form ,when the user enter the
field in textbox after fill up user hit the enter button on the keyboard Then the deafult button will fire..
let me see where to write default button coding ...

<form id="form1" runat="server" defaultbutton ="button3">


In the main form id you have to add the default button id...