From Ad Hacker Wiki

Javascript is a computer language that can be used to write programs in web pages. When the web page is shown on your computer, your computer also runs these Javascript programs.

Javascript programs in web pages can be very simple, such as validating forms to make sure you only entered digits for a telephone number, or very complex such as Google Docs which provides a full spreadsheet and word processor within your browser. Javascript can watch the keys you type and how you move your mouse, and send that information across the Internet to a computer - this is the part most people don't know about.