jQuery is a JavaScript library of functions, designed to make JavaScript much easier to use. It has useful functions for manipulating HTML, animation, AJAX handling and much more. jQuery wraps around the base JavaScript code and abstracts away many of the foibles and pitfalls that make JavaScript difficult to work with. Here’s a typical example which shows how jQuery greatly simplifies JavaScript:
jQuery:

$.ajax({
    type: 'POST',
    url: 'example.com',
    data: data
});

Equivalent JavaScript:

var request = new XMLHttpRequest();
request.open('POST', 'example.com', true);
request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');
request.send(data);

jQuery is the most popular JavaScript library in use and one of the most popular web technologies, being used in almost three quarters of all websites. It is so ubiquitous that many developers write commands in jQuery without ever learning the equivalent code in JavaScript.

A guy named John Resig created the first release of jQuery in 2006. Nowadays, it is open source software meaning it is free to use and anyone can contribute to it’s codebase. It is actively worked on with version 3.1 being released in July 2016.

jQuery Plugins

As it extends JavaScript, jQuery can itself be extended by plugins of which there are thousands. Below are some notable examples:

• jQueryUI which adds slick user-friendly user interface elements.
• jQuery Mobile for making responsive websites.
• jQuery Validate for validating form data entered by users.

Read More

Find out more about jQuery at the jQuery Wikipedia article.