You can use AJAX, HTML, CSS.
Flash, Illustrator and a lot of other scripting languages that are commonly used nowadays.
It was introduced with Netscape Navigator for website development and is designed to build dynamic online pages, used to check details on html forms before the page is allowed to submit any data.
If you are into website development, of course you need your visitors to be able to interact with the website dynamically as nobody likes a stale webpage that just sit there.
It is also helpful in protecting your passwords, detecting browsers and displaying information.
Remember that testing is essential in website development to make sure that you are getting your desired output.