"jQuery is a lightweight JavaScript library that emphasizes interaction between JavaScript and HTML." ~ According to Google

"jQuery is a fast and concise JavaScript Library that simplifies HTML document traversing, event handling, animating, and Ajax interactions for rapid web development. jQuery is designed to change the way that you write JavaScript."

http://jquery.com/

Basically, JQuery is a lightweight, modular object oriented programming Javascript framework. It simplifies event handling, animations, and Ajax interactions for web development. It allows you to manipulating html with very little code and you can then chain these interactions together so if one event occurs, the value can be stored and used to give life to the next command.