JavaScript is a scripting language that allows the web page author to control certain aspects of the browser and web pages. You might want to use it to create drop-down menus, perform form validation, or even to produce a small program. I wrote the Due Date Calculator for an Ob/Gyn group. Secret Message II lets you build or solve coded messages.
The drop-down menus on this site use CSS with just a speck of javascript. I wrote a little script that randomly chooses from a number of photos and places one at the top of the page when it loads.