jQuery is a library of JavaScript functions that can be accessed using very little code. The real purpose behind jQuery is to write JavaScript code quickly. If you are new to web programming, jQuery can be easier to learn once you have mastered basic knowledge of HTML, CSS and JavaScript. More importantly – understanding of the Document Object Model or “the DOM” .

A good place to start learning jQuery is Try JQuery from the Code School. It is a series of video based learning tutorials with challenges at each level to test your understanding of some of the basic jQuery functions.

More detailed information can be found at the official JQuery site

Another great source for learning jQuery as well as HTML, Javascript and CSS can be found in the following books  – Javascript & jQuery by Jon Puckett

JQueryBook

and Learning Web Design: A Beginner’s Guide to HTML, CSS, JavaScript, and Web Graphics

WebProgrammingBook