Create a file hello.html with the following content:

<!DOCTYPE html>
<html>
<head>
    <title>Hello, World!</title>
</head>
<body>
    <div>
        <p id="hello">Some random text</p>
    </div>
    <script src="https://code.jquery.com/jquery-2.2.4.min.js"></script>
    <script>
        $(document).ready(function() {
            $('#hello').text('Hello, World!');
        });
    </script>
</body>
</html>

Live Demo on JSBin (opens new window)

Open this file in a web browser. As a result you will see a page with the text: Hello, World!

Explanation of code

<script src="https://code.jquery.com/jquery-2.2.4.min.js"></script>

// When the `document` is `ready`, execute this function `...`
$(document).ready(function() { ... });
 
// A commonly used shorthand version (behaves the same as the above)
$(function() { ... });

Libraries other than jQuery may also use $ as an alias. This can cause interference between those libraries and jQuery.

To release $ for use with other libraries:

jQuery.noConflict();

After calling this function, $ is no longer an alias for jQuery. However, you can still use the variable jQuery itself to access jQuery functions:

jQuery('#hello').text('Hello, World!');

Optionally, you can assign a different variable as an alias for jQuery:

var jqy = jQuery.noConflict();
jqy('#hello').text('Hello, World!');

Conversely, to prevent other libraries from interfering with jQuery, you can wrap your jQuery code in an immediately invoked function expression (IIFE) (opens new window) and pass in jQuery as the argument:

(function($) {
    $(document).ready(function() {
        $('#hello').text('Hello, World!');
    });
})(jQuery);

Inside this IIFE, $ is an alias for jQuery only.

Another simple way to secure jQuery's $ alias and make sure DOM is ready:

jQuery(function( $ ) { // DOM is ready
   // You're now free to use $ alias
   $('#hello').text('Hello, World!');
});

To summarize,

• jQuery.noConflict() : $ no longer refers to jQuery, while the variable jQuery does.
• var jQuery2 = jQuery.noConflict() - $ no longer refers to jQuery, while the variable jQuery does and so does the variable jQuery2.

Now, there exists a third scenario - What if we want jQuery to be available only in jQuery2? Use,

var jQuery2 = jQuery.noConflict(true)

This results in neither $ nor jQuery referring to jQuery.

This is useful when multiple versions of jQuery are to be loaded onto the same page.

<script src='https://code.jquery.com/jquery-1.12.4.min.js'></script>
<script>
    var jQuery1 = jQuery.noConflict(true);
</script>
<script src='https://code.jquery.com/jquery-3.1.0.min.js'></script>
<script>
    // Here, jQuery1 refers to jQuery 1.12.4 while, $ and jQuery refers to jQuery 3.1.0.
</script>

https://learn.jquery.com/using-jquery-core/avoid-conflicts-other-libraries/ (opens new window)

jQuery is the starting point for writing any jQuery code. It can be used as a function jQuery(...) or a variable jQuery.foo.

$ is an alias for jQuery and the two can usually be interchanged for each other (except where jQuery.noConflict(); has been used - see Avoiding namespace collisions (opens new window)).

Assuming we have this snippet of HTML -

<div id="demo_div" class="demo"></div>

We might want to use jQuery to add some text content to this div. To do this we could use the jQuery text() (opens new window) function. This could be written using either jQuery or $. i.e. -

jQuery("#demo_div").text("Demo Text!");

Or -

$("#demo_div").text("Demo Text!");

Both will result in the same final HTML -

<div id="demo_div" class="demo">Demo Text!</div>

As $ is more concise than jQuery it is the generally the preferred method of writing jQuery code.

jQuery uses CSS selectors and in the example above an ID selector was used. For more information on selectors in jQuery see types of selectors (opens new window).

Sometimes one has to work with pages that are not using jQuery while most developers are used to have jQuery handy.

In such situations one can use Chrome Developer Tools console (F12) to manually add jQuery on a loaded page by running following:

var j = document.createElement('script');
j.onload = function(){ jQuery.noConflict(); };
j.src = "https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js";
document.getElementsByTagName('head')[0].appendChild(j);

Version you want might differ from above(1.12.4) you can get the link for one you need here (opens new window).

To load jQuery from the official CDN (opens new window), go to the jQuery website (opens new window). You'll see a list of different versions and formats available.

(opens new window)

Now, copy the source of the version of jQuery, you want to load. Suppose, you want to load jQuery 2.X, click uncompressed or minified tag which will show you something like this:

(opens new window)

Copy the full code (or click on the copy icon) and paste it in the <head> or <body> of your html.

The best practice is to load any external JavaScript libraries at the head tag with the async attribute. Here is a demonstration:

<!DOCTYPE html>
    <html>
      <head>
         <title>Loading jquery-2.2.4</title>
         <script src="https://code.jquery.com/jquery-2.2.4.min.js" async></script>
      </head>
      <body>
          <p>This page is loaded with jquery.</p>
      </body>
   </html>

When using async attribute be conscious as the javascript libraries are then asynchronously loaded and executed as soon as available. If two libraries are included where second library is dependent on the first library is this case if second library is loaded and executed before first library then it may throw an error and application may break.

Every time jQuery is called, by using $() or jQuery(), internally it is creating a new instance of jQuery. This is the source code (opens new window) which shows the new instance:

// Define a local copy of jQuery
jQuery = function( selector, context ) {

    // The jQuery object is actually just the init constructor 'enhanced'
    // Need init if jQuery is called (just allow error to be thrown if not included)
    return new jQuery.fn.init( selector, context );
}

Internally jQuery refers to its prototype as .fn, and the style used here of internally instantiating a jQuery object allows for that prototype to be exposed without the explicit use of new by the caller.

In addition to setting up an instance (which is how the jQuery API, such as .each, children,filter, etc. is exposed), internally jQuery will also create an array-like structure to match the result of the selector (provided that something other than nothing, undefined, null, or similar was passed as the argument). In the case of a single item, this array-like structure will hold only that item.

A simple demonstration would be to find an element with an id, and then access the jQuery object to return the underlying DOM element (this will also work when multiple elements are matched or present).

var $div = $("#myDiv");//populate the jQuery object with the result of the id selector
var div = $div[0];//access array-like structure of jQuery object to get the DOM Element

Typically when loading plugins, make sure to always include the plugin after jQuery.

<script src="https://code.jquery.com/jquery-3.1.1.min.js"></script>
<script src="some-plugin.min.js"></script>

If you must use more than one version of jQuery, then make sure to load the plugin(s) after the required version of jQuery followed by code to set jQuery.noConflict(true) (opens new window); then load the next version of jQuery and its associated plugin(s):

<script src="https://code.jquery.com/jquery-1.7.0.min.js"></script>
<script src="plugin-needs-1.7.min.js"></script>
<script>
// save reference to jQuery v1.7.0
var $oldjq = jQuery.noConflict(true);
</script>
<script src="https://code.jquery.com/jquery-3.1.1.min.js"></script>
<script src="newer-plugin.min.js"></script>

Now when initializing the plugins, you'll need to use the associated jQuery version

<script>
// newer jQuery document ready
jQuery(function($){
  // "$" refers to the newer version of jQuery
  // inside of this function

  // initialize newer plugin
  $('#new').newerPlugin();
});

// older jQuery document ready
$oldjq(function($){
  // "$" refers to the older version of jQuery
  // inside of this function

  // initialize plugin needing older jQuery
  $('#old').olderPlugin();
});
</script>

It is possible to only use one document ready function to initialize both plugins, but to avoid confusion and problems with any extra jQuery code inside the document ready function, it would be better to keep the references separate.

Remarks

jQuery is a JavaScript library which simplifies DOM operations, event handling, AJAX, and animations. It also takes care of many browser compatibility issues in underlying DOM and javascript engines.

Each version of jQuery can be downloaded from https://code.jquery.com/jquery/ (opens new window) in both compressed (minified) and uncompressed formats.