Solved: javascript event handlers

Can someone explain whats happening in the followong lines of code, taken from the application attached..

var $ = function ( id ) {
return document.getElementById( id );
}
$("btnDisplay").onclick = display_click; // In particular, what is going on here??

Thanks

head>
<title>JavaScript Event Handler</title>
<script type="text/javascript">
// This function receives an id and gets the related XHTML object.
var $ = function ( id ) {
return document.getElementById( id );
}
var display_click = function () {
alert( "You clicked the button.");
}
window.onload = function () {
$("btnDisplay").onclick = display_click;
}
</script>
</head>
<body>
<p><input type="button" value="Display Message" id="btnDisplay" /></p>
</body>
</html>

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