Java Script is set of instructions, codes or components used to create "dinamic" web pages, As example a Java Script could be used for updating in real time data or info from a module or even to show us an image when pointing the mouse over a link into a web site. Java Script is a really important part of the actual "Web 2.0" platform and runs on almost all web sites (if not all) that in any way keeps some interactivity with a user.
To lear more about Java Script visit
http://www.w3schools.com/js/default.asp or take a look at the Wiki following:
http://en.wikipedia.org/wiki/JavaScript where you can see that J ava Script and Java Programming Language are two different things (that of course can work together).