What is jQuery ?

Released in January of 2006, jQuery is a cross-browser JavaScript library designed to simplify the client-side script of HTML.
Used by close to a million websites, jQuery is the most popular JavaScript library in use to date.
jQuery makes it easy to handle DOM objects, events, effects and Ajax, automatically takes care of JavaScript leaks, and has countless third-party plugins. Read more on Wikipedia.

For a complete Learning on JQuery visit Web Technologist- level 1.