JavaScript is a scripting language that you can embed in your HTML to add interactivity to your site. For example, the Postcrypt Art Gallery website uses JavaScript for the artwork archive. When you click on a show title, work and a description of the show appears on the left side and when you mouse-over the thumbnails, you can see larger versions of the artwork. This page is one HTML document with CSS & JavaScript controlling the design and interactivity.
JavaScript is an object-oriented language, which is different from CSS or HTML beacuse it handles events and adds the possibility of dynamic content to your webpage. If you would like to learn some JavaScript, try the W3Schools JavaScript Tutorial.
Macromedia Flash is a professional program that can be used to create interactive websites. The Flash application is embedded into the HTML file. Macromedia Flash is an expensive piece of software, but if you have access to it, it can be a great tool. Below are some examples of websites made with Flash: