.val()

$('select.foo option:selected').val();    // get the value from a dropdown select
$('select.foo').val();                    // get the value from a dropdown select even easier
$('input:checkbox:checked').val();        // get the value from a checked checkbox
$('input:radio[name=bar]:checked').val(); // get the value from a set of radio buttons

<script>
    $("button").click(function () {
      var text = $(this).text();
      $("input").val(text);
    });
</script>