Definition:
jQuery is a library that makes it easy and faster to create web pages and applications with JavaScript. jQuery makes use of many common tasks that require lines of JavaScript code and wraps them in methods that can be summarized with a single line of code. Put more simply, JQuery writes in a line of code what JavaScript would have written in 10 or 20 lines.
It can be downloaded for free. In addition to the JQuery library, there are plenty of jQuery plugins that are available to add even more power and functionality to your scripts.
What you can do with jQuery
With jQuery you can add animated effects to the elements (fade, output and expansion, etc.). It also allows you to make XMLrequests, manipulate the DOM, create slideshows and drop-down menus, among many other things.
Another of jQuery’s strengths is that it makes it easy to write JavaScript that works in many different browsers. That is, with JavaScript it is sometimes necessary to write different lines of code for each browser, but with jQuery all you have to do is call the right function so that the code runs in different browsers.