jQuery is a very lightweight Javascript library (some call it a framework), that takes most of the headache out from writing pure Javascript in your applications. It has many very powerful features, some of which include: easily traversing the DOM, adding slick animations and effects to elements, and super simple Ajax techniques and methods.
Learning jQuery - Tips, Techniques, Tutorials
jQuery Demo - Expandable Sidebar Menu on Vimeo
Create a jQuery slideshow that enables you to click through each slide when JavaScript is disabled, without having to display all slides one under the other.
15 Days Of jQuery Examples and tutorials to help you learn JQuery
51+ Best of jQuery Tutorials and Examples « Noupe