JQuery :: How To Check Value Of Input Box

I would like to check the value in an input box and if the value has changed, then perform the action. Being a newbie at Jquery, I am not sure about the syntax.
var timeout = undefined;
var str = ''; /* Get the initial value and if changed, do something */
$('#input_box').live('keyup', function() {
if(timeout != undefined) {
clearTimeout(timeout);
}
var timeout = setTimeout(function() {
timeout = undefined;
/* Do something */
$('#log').append('<div>Handler for input_box .keyup() called.</div>');
}, 1500);
});