About jQuery 

jQuery takes a lot of common tasks that require many lines of JavaScript code to accomplish,
and wraps them into methods that you can call with a single line of code.

jQuery also simplifies a lot of the complicated things from JavaScript, like AJAX calls and DOM manipulation.

The jQuery library contains the following features:

HTML/DOM manipulation
CSS manipulation
HTML event methods
Effects and animations
AJAX
Utilities

.

Why jQuery?

There are lots of other JavaScript libraries out there, but jQuery is probably the most popular, and also the most extendable.

Many of the biggest companies on the Web use jQuery, such as:

Google
Microsoft
IBM
Netflix

Adding jQuery to Your Web Pages
There are several ways to start using jQuery on your web site. You can:

Download the jQuery library from jQuery.com
Include jQuery from a CDN, l
ike Google

Downloading jQuery
There are two versions of jQuery available for downloading:

Production version – this is for your live website because it has been minified and compressed
Development version – this is for testing and development (uncompressed and readable code)
Both versions can be downloaded from jQuery.com.

The jQuery library is a single JavaScript file, and you reference it with the HTML

Google CDN: