JavaScript is a scripting language which can be included directly into HTML pages. It enhances user interface and functionality of websites.
JavaScript can be used for adding interactivity to Web content.
JavaScript to use writing code for validating forms, totaling orders, handling image rollovers and other display tasks, conditionally navigating the user to various pages, and more.
A scripting language is a lightweight programming language. JavaScript is programming code that can be inserted into HTML pages. JavaScript inserted into HTML pages, can be executed by all modern web browsers.

Here are few things that you can easily do with JavaScript:
1.Add Scrolling message to Browser’s status line.
2.Menus which open-up on mouse-over (e.g. Koenig's Left Menu)
3.Display message to user.
4.Forms Validation.
5.Create ad banners that can interact with the user.
6.Detect the browser in use and display appropriate contents according to browser.