JavaScript is an easy-to-use programming language which runs client-side (on each web surfer's computer) and adds useful or decorative interactivity to web pages. No extra plug-ins need to be installed into the browser for JavaScript to run and, although it can be disabled by users to stop script running, it is safe to assume that any JavaScript code added to your pages will run on the vast majority of the computers of visitors to your site. Dreamweaver allows you to add JavaScript to your code automatically and easily by using "behaviors".