JavaScript is a platform-independent, event-driven, interpreted programming language developed by Netscape and Sun Microsystems. Originally called LiveScript, JavaScript is affiliated with Sun's object-oriented programming language JavaTM primarily as a marketing convenience. They interoperate well but are technically, functionally and behaviorally very different.

JavaScript is useful for adding interactivity to your web site because scripts can be embedded in HTML files simply by enclosing code in a <script> </script> tag pair. All modern day browsers can interpret JavaScript, albeit with some irritating caveats. In practice, JavaScript is a fairly universal extension to HTML that can enhance the user experience through event handling and client-side execution, while extending a web developer's control over the client's browser.

To begin using JavaScript, you will need to know how to use HTML and be able to edit your HTML documents. Here are some links free Javascript resources and tutorials.

QuirckMode
Contains about 120 pages with CSS and JavaScript tips and tricks, and is one of the best sources on the web for studying and defeating browser incompatibilities.

JavaScript Source
JavaScript Source is an excellent JavaScript resource with tons of cut and paste JavaScript examples for your web pages.

Javascript The Definitive Resource
Tutorials and free example JavaScripts you can cut and paste into your HTML documents.

W3Schools Javascript Tutorials
W3Schools is one of the definitive resources on the web when it comes to site development, they have extensive tutorials on HTML, ASP, PHP, XML and JavaScript.