What is JQuery? · Jquery is a light weight java script library. · JQuery is cross browser. · Easy to write compare to java script. $(document).ready is a jquery event. It fires as soon as the DOM is loaded. This event is fired before all the images, css are fully loaded. $(window).load event fires when DOM and all content (images, css) loads. What is DOM? The HTML DOM (Document Object Model), when a web page is loaded, the browser creates a Document Object Model of that page. The HTML DOM model is constructed as a tree of Objects like HTML, HEAD, Body, H1, P etc.