Как работает load jquery?

var loadScript = function(src, callback, appendTo) {
    var script = document.createElement('script');

    if (!appendTo) {
        appendTo = document.getElementsByTagName('head')[0];
    }

    if (script.readyState && !script.onload) {
        // IE, Opera
        script.onreadystatechange = function() {
            if (script.readyState == "loaded" || script.readyState == "complete") {
                script.onreadystatechange = null;
                callback();
            }
        }
    }
    else {
        // Rest
        script.onload = callback;
    }

    script.src = src;
    appendTo.appendChild(script);
}