Javascript is a platform independent program which you can use in your HTML files. You can do lots of useful things to spice up your visitors view experience. While an entire site can be built on
JS, most people use the scripts with HTML for nifty utilities, like, feedback form verification, showing current date, image change on mouse-over links etc. You will get millions of free javascripts for your web site provided by thousands of web sites. Just search for "free javascripts" in yahoo, and you will find them all. Just cut-paste the code in the desired section in your HTML and you are ready to go!