jQuery code is often wrapped in jQuery(function($) { ... }); so that it only runs after the DOM has finished loading.

<script type="text/javascript"> 
  jQuery(function($) {
    // this will set the div's text to "Hello".
    $("#myDiv").text("Hello");
  });
</script>

<div id="myDiv">Text</div>

This is important because jQuery (and JavaScript generally) cannot select a DOM element that has not been rendered to the page.

<script type="text/javascript">
  // no element with id="myDiv" exists at this point, so $("#myDiv") is an
  // empty selection, and this will have no effect
  $("#myDiv").text("Hello");
</script>

<div id="myDiv">Text</div>

Note that you can alias the jQuery namespace by passing a custom handler into the .ready() method. This is useful for cases when another JS library is using the same shortened $ alias as jQuery, which create a conflict. To avoid this conflict, you must call $.noConflict(); - This forcing you to use only the default jQuery namespace (Instead of the short $ alias).
By passing a custom handler to the .ready() handler, you will be able to choose the alias name to use jQuery.

$.noConflict();

jQuery( document ).ready(function( $ ) {
   // Here we can use '$' as jQuery alias without it conflicting with other 
   // libraries that use the same namespace 
   $('body').append('<div>Hello</div>')
});

jQuery( document ).ready(function( jq ) {
   // Here we use a custom jQuery alias 'jq' 
   jq('body').append('<div>Hello</div>')
});

Rather than simply putting your jQuery code at the bottom of the page, using the $(document).ready function ensures that all HTML elements have been rendered and the entire Document Object Model (DOM) is ready for JavaScript code to execute.

These are all equivalent, the code inside the blocks will run when the document is ready:

$(function() {
  // code
});

$().ready(function() {
  // code
});

$(document).ready(function() {
  // code
});

Because these are equivalent the first is the recommended form, the following is a version of that with the jQuery keyword instead of the $ which produce the same results:

jQuery(function() {
  // code
});

Notation

As of jQuery 3.0, only this form is recommended:

jQuery(function($) {
  // Run when document is ready
  // $ (first argument) will be internal reference to jQuery
  // Never rely on $ being a reference to jQuery in the global namespace
});

All other document-ready handlers are deprecated in jQuery 3.0. (opens new window)

Asynchronous

As of jQuery 3.0, the ready handler will always be called asynchronously (opens new window). This means that in the code below, the log 'outside handler' will always be displayed first, regardless whether the document was ready at the point of execution.

$(function() {
  console.log("inside handler");
});
console.log("outside handler");

> outside handler
> inside handler

$(window).load() was deprecated in jQuery version 1.8 (and completely removed from jQuery 3.0 (opens new window)) and as such should not be used anymore. The reasons for the deprecation are noted on the jQuery page about this event (opens new window)

Caveats of the load event when used with images A common challenge developers attempt to solve using the .load() shortcut is to execute a function when an image (or collection of images) have completely loaded. There are several known caveats with this that should be noted. These are:

• It doesn't work consistently nor reliably cross-browser
• It doesn't fire correctly in WebKit if the image src is set to the same src as before
• It doesn't correctly bubble up the DOM tree
• Can cease to fire for images that already live in the browser's cache

If you still wish to use load() it is documented below:

$(document).ready() waits until the full DOM is availble -- all the elements in the HTML have been parsed and are in the document. However, resources such as images may not have fully loaded at this point. If it is important to wait until all resources are loaded, $(window).load() and you're aware of the significant limitations of this event then the below can be used instead:

$(document).ready(function() {
  console.log($("#my_large_image").height()); // may be 0 because the image isn't available
});

$(window).load(function() {
  console.log($("#my_large_image").height()); // will be correct
});

Example uses of $(document).ready():

$(document).ready(function() {
  $("button").click(function() {
    // Code for the click function
  });
});

1. Run jQuery code after the page structure is created

jQuery(function($) {
// set the value of an element.
   $("#myElement").val("Hello");
});

$(document).ready(function() {
  $("#toggleDiv").hide();
  $("button").click(function() {
    $("#toggleDiv").show();
  });
});

Using the document-ready event can have small performance drawbacks (opens new window), with delayed execution of up to ~300ms. Sometimes the same behavior can be achieved by execution of code just before the closing </body> tag:

<body>
  <span id="greeting"></span> world!
  <script>
    $("#greeting").text("Hello");
  </script>
</body>

will produce similar behavior but perform sooner than as it does not wait for the document ready event trigger as it does in:

<head>
  <script>
    jQuery(function($) {
      $("#greeting").text("Hello");
    });
  </script>
</head>
<body>
  <span id="greeting"></span> world!
</body>

Emphasis on the fact that first example relies upon your knowledge of your page and placement of the script just prior to the closing </body> tag and specifically after the span tag.