Timing differences between $(document).ready() and window.onload()

Introduction
In JavaScript, window.onload event fires when a document is completely downloaded to the browser. This means that every element on the page is ready to be manipulated by JavaScript, which is a boon for writing featured code without worrying about load order.

On the other hand, jQuery's $(document).ready() is invoked when the DOM is completely ready for use. This also means that all elements are accessible by our scripts, but does not mean that every associated file has been downloaded. As soon as the HTML has been downloaded and parsed into a DOM tree, the code can run.

Example

Think of a page that presents an image gallery; such a page may have many large images on it, which we can hide, show, move, and otherwise manipulate with jQuery. If we set up our interface using the onload event, users will have to wait until each and every image is completely downloaded before they can use those features. Even worse, if behaviors are not yet attached to elements that have default behaviors (such as links), user interactions could produce unintended outcomes.

So, when we use $(document).ready() for the setup, the interface is ready to use earlier with the correct behavior.
Read next post for its real example.

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