What is jQuery?
jQuery is a lightweight, "write less, do more", JavaScript library.
The purpose of jQuery is to make it much easier to use JavaScript on your website.
jQuery takes a lot of common tasks that requires many lines of JavaScript code to accomplish, and wraps it 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

Tip: In addition, jQuery has plugins for almost any task out there.