What is the main difference between window.onload and onDocumentReady?

Both functions are used to perform tasks when the page is loaded in the browser but they have important differences. Most importantly, "window.onload" will execute code when browser has loaded the DOM tree and all other resources like images, objects, etc, while onDocumentReady allows you to execute code when only the DOM tree has been built, without waiting for images to load. Thus, you can start scripting agains the DOM much quicker with onDocumentReady. Another important difference is that window.onload is not cross-browser compatible while using something like jQuery's document.ready() will work nicely on all browsers.