JavaScript is the glue that connects user interaction with the server side. Any kind of special effects that happen in the browser are all done using JavaScript (DHTML). The exception to this is plugins like Flash, mind you a lot of Flash objects need JavaScript to get started, e.g. SWFObject.
Below are some links to some JavaScript resources. This list will grow over time. I also highly recommend reading Douglas Crockford's book, JavaScript: The Good Parts.