A page can't be manipulated safely until the document is "ready." jQuery detects this state of readiness for you. Code included inside $( document ).ready() will only run once the page Document Object Model (DOM) is ready for JavaScript code to execute. Code included inside $( window ).on( "load", function() { ... }) will run once the entire page (images or iframes), not just the DOM, is ready.

// A $( document ).ready() block.
$( document ).ready(function() {
    console.log( "ready!" );
});

Experienced developers sometimes use the shorthand $() for $( document ).ready(). If you are writing code that people who aren't experienced with jQuery may see, it's best to use the long form.

// Shorthand for $( document ).ready()
$(function() {
    console.log( "ready!" );
});

You can also pass a named function to $( document ).ready() instead of passing an anonymous function.

// Passing a named function instead of an anonymous function.
function readyFn( jQuery ) {
    // Code to run when the document is ready.
}
$( document ).ready( readyFn );
// or:
$( window ).on( "load", readyFn );

The example below shows $( document ).ready() and $( window ).on( "load" ) in action. The code tries to load a website URL in an <iframe> and checks for both events:

<html>
<head>
    <script src="https://code.jquery.com/jquery-1.9.1.min.js"></script>
    <script>
    $( document ).ready(function() {
        console.log( "document loaded" );
    });
    $( window ).on( "load", function() {
        console.log( "window loaded" );
    });
    </script>
</head>
<body>
    <iframe src="http://techcrunch.com"></iframe>
</body>
</html>

To learn more about .ready() method, you can read the relevant documenation page.